There have been title tests of the pixel count to 111 characters, but to me that's way too much! Get it concise without making it into a mini advertorial infomercial!
Google's search downstream partners do clip some of the title and meta-description by 2-4 characters. I tend to use a 68 characters limit for title and 145 characters for meta-description.
By all means test, test and test. Local ISP's search pages, many meta search portals and even AOL clip their data to be shorter. If these pages do NOT matter use your test results to push out the length to what's stated above by aakk9999. This clipping typically takes out the last word - if your last word is longish and goes over the limit by ONE character you loose the whole word (replaced by ...).
This length choice (and the last word[s] clipping) that makes writing naturally sounding ones even more challenging! Some ISP's snag url typos with their DNS and redirect all errors to their search portal. Comcast allows you to opt-out of this....others vary. Some use Google, some Yahoo and a few AOL. Typo a URL to test!
facebook will use 297 characters (of the meta-description) if no Open Graph 'og:description' tag is present.
You think it's BOTH, first the number of characters and then additionally by pixels?
From when I last time played with this, the limit seemed to be 155 characters. I had a page where meta description had 153 characters, but the second line ended up somewhere just after the middle. I tried to add an additional couple of words (that would have fitted according to pixels), but the words that I added got cut off.
Having said this, I have just checked randomly a number of meta descriptions in SERPs and I found some that are going to 164 characters, so perhaps now it has changed to pixels.
On pages that might rank on a range of terms, I sometimes use a "150/250" arrangement, as I call it... 250 characters overall, with roughly the first 150 characters including the most-likely-to-rank terms. These 150 are incorporated into a self-contained thought to display as a sentence; and I play with the remaining 100 characters to add secondary targets.
Google seems to be good at spotting break points, particularly when they're indicated with punctuation. Semicolons can be useful to break descriptions; they're more flexible than periods or commas.
For the information of newbies who might be reading this... while the meta description doesn't affect rankings, the highlighted search terms in the results can attract the eye and increase click throughs, particularly when combined with an effective call to action or value proposition.
@Hoople: I'd like to avoid test a lot. One reason Robert and JS_Harris mentioned: Google sometimes breaks the basic rules on purpose, so you can't be sure for your testing results. Another reason is having time....
It's not necessary for me to have a very big description. I just like to have a good short description which is shown as often as possible and not shortened by dots. The dots don't look very teasing to users... so "incomplete" and "lacking"... bad subconscious psycological effect...
The title tag obviously is only determined by pixels and you can check this by Google itsself! Use the testing tool for structured data by inserting the whole title tag as code in the html tab. Unfortunately this doesn't work with the description, at least not for me, but check it.
@mromero: Nice idea, but making curious is already the task of the whole decription AND furthermore promising to fulfill awakened needs/interests. I'd like to accomplish this both completely.
@Robert: Nice idea, having a plan A and B at the same time. If Google shows the first 150 characters you don#t have shortening dots?
Why do you think do i want to know the exact length? Because I'm a prof of mathematics, fallen in love with naked numbers?
No, it should help me to use the place as good as possible and to avoid the ugly cut with dots. So it should help me showing a good description which people like to click.
I guess the number of characters depend on how good you fill the first row in terms of pixels. If there are some small words at the end of the first row, like "it is a top...", then you will fill it good and get more characters in total.
I use a maximum of 156 characters (including spaces). Any longer than this and you risk your description being truncated. However, for some queries Google will show a larger meta description. Also, if Google picks up other things on your page, the meta description that you have crafted may be severely truncated.
I would kindly suggest re-reading Robert Charlton's advice about the 150/250 character arrangement, and test it out. This is EXTREMELY helpful when a certain page might rank for two different sets of keywords.
Pretty much the only time Google will show your meta description is if all of the keywords in the user's search phrase are found in the meta description. It's why I optimize my title element first (should contain the 1-3 keyword phrases your URL is targeting and most likely to rank for if you're doing your job well).
Once I know what my title element looks like I then optimize my meta description so that it uses hopefully most if not all of the individual keywords from the title. This should maximize the percentage of time that Google actually displays your meta description as the snippet.
The only time Google seems to show more than 150 or so characters from your meta description is if the search phrase it ranks for is a very long tail keyword phrase (typically 5 or more keywords in the phrase, maybe 4+ in some instances). Then and only then might they show 200, 300, even 400 characters from the meta description.
So if you're targeting primarily 1-3 word phrases, then I would stick to 145-150 as the max length to avoid the unattractive elipses [...] at the end where Google truncates it. If your going after very long search phrases then you might consider something longer, though 150 characters is almost always enough to have 1-2 sentences, each starting with a call to action and using most if not all keyword phrases from the title element. So I honestly have never needed more than 150.
@Planet13: Yes, indeed, Robert's idea is very interesting and creative (that's what really good webmasters and SEOs characerizes IMHO).
But I guess then there are still the truncating dots and there is another "problem": Usually a page shouldn't be optimized for two sets of KWs. Two or three relating KWs/phrases o.k., but two SETS of KWs usually is too much and not a good idea. Then better creating two pages in order to better focuse on both parts.
You cannot rank with all KWs, you have to make a decision and concentrate (onpage and offpage). IMHO this problem is only neglectable if choosing a "second class" KW or phrase which has very few competition and you can rank without many backlinks and onpage love.
Just my 2cents...
@ZydoSEO: This makes sense and that's exactly what I'm doing. Make the description as relevant as possible for as many KWs as possible. It makes it more difficult to create teasing and clickable sentences when you have to use several KWs/longtail. But if you at least accomplish to create readable sentences with your KWs.... I think you cannot do more to encourage Google to use your description. Right?
And because it's more difficult to create tempting sentences with several pre-determined KWs/longtails/phrases which "must" appear, it is important to use the place as effective as possible. Therefore my question...and the circle is complete.