The 250 characters is a vague guideline rather than a rule of law. Most search engines give little or no weight to the meta keywords tag anyway, so I wouldn't worry too much about it. If you want to use it, just put five or six relevant keywords and leave it at that. You can then spend your time on adding the much more valuable on-page content instead!
You should use either a space or a comma and a space for separating the keywords.
Yes, "characters" includes any commas or spaces. Back in the day when the keyword meta tag was heavier in the search engine algos, I had better results using a space as a separator rather than the comma (and never both because that's just wasteful!)
Using a simple space as a separator allowed a much more free phrase match - the comma was seen as more of a "hard" separation, so kw1,kw2 was not the same as kw1 kw2. Because of this some people ended up with kw1,kw2, kw1 kw2, just to get the two word phrase to rank as well as the single words...and they rapidly ran out of characters that way.
I've kept that same habit today (using spaces only as separators) even though the keyword tag means very little today, as encyclo and Wlauzon have said above.