Here is a great site with a lot of FAQ you might want asnwers to: The link is directly to the keg physical sizes FAQ, but there are a lot more – including CO2 cylinder information.
http://ceisites.com/tap_info.htm#keg_types
Oh, I forgot to mention. For the most part, unless you are getting domestic beers, you will have to buy a regular 1/2 keg. In fact, even for many good domestic (especially microbrews) you can not even get the 1/4 or 1/6 kegs. You generally can order kegs from your local distributor that are not listed on their available beer list if you locate the brewer and the beer you want to get.