aakk9999 - 1:18 am on Jul 25, 2013 (gmt 0)
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.
Also, I like the answer Hoople gave above.