Hi Mark,
Currently the field in the database is nvarchar(255), so that is all that is going to fit.
I will put it on my list to see about increasing the size of that field but it is not a high priority.
As far as I know meta keywords are dead, and not used by google, yahoo or Bing. You are far better off using the keywords in real content instead of in the meta. I think stuffing a lot of keywords into meta especially if they are not related to actual content on the page can hurt SEO and the reputation of your site.
Nevertheless, when I get a chance I will see about enlarging that field.
Best,
Joe