It all depends on the search engine that is indexing your page, so in theory it could be infinite times long.
As a rule of thumb though, the major search engines (Google, Yahoo and Bing) tend to accept around 150 characters for the description, and about 65 for the title.
I don't believe Google uses the Meta Keyword Tag and instead uses words on the pages themselves, but other search engines do. I would suggest about 15-20 meta keywords.
In terms of what tags you should include, depends on your site. Make sure you include words which are unique to your site and avoid generic terms.
If something only exists on your site, make sure you include this so that people searching for your product find your website.
There is no such limitation of tags so you can use many tags you wants.
Although there is no such limitation, search engines only index a certain amount of meta data and the rest is ignored. It is therefore important that you place your most important meta content in the first X amount.