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Certainly make sure your main keywords are listed in there first to give them a boost, and it is a good place to put manufacturers' names, specific product names etc. as a search engine might draw on your meta keywords if these highly specific words can't be found in the content of other pages.
As for length - I have no worries about the length of my meta keywords - there are some highly ranked pages with hundreds of words in there, in all combinations.
Probably because that particular SE ignores the tag. They could have a thousand words in there and it would not matter.
Many say the keywords tag is dead. I'm not convinced yet. I still use them on primary pages and will typically keep them to about 10, 15, or 20 words maximum. I suggest that you not repeat the same word more than three times. There should also be space between those repeats, for example...
blue widgets california red widgets orange county green widgets southern california
Notice how I have a nice spread balance between the word widgets which is also repeated three times.
Your keywords tag should come from your title, description, main heading and first paragraph on the page. The goal is to tie those elements together.
Also, utilizing your metas properly is just good design practice. First thing I do when building a page is format my title, description, keywords, and sytle/javascript external file references. I also use link relationship metas where applicable.