I voted for 20 but most likely the most common size is 5 and under. Think about all those beta and goldfish bowls out there. But most people that have them will not come to this site for info and therefore they won't vote.
Most people don't consider anything under 5 gallons a tank but rather a bowl. 10 gallons is the most popular. Thats why they sell a lot of 10 gallon setups. Cheap and they get you started. 5 gallon tanks cost as much if not more than 10 gallon ones.
I would have to say 10, but I can't really make a choice. In tanks from 5 gal up, I have 1 five, 10 ten gallon tanks, 1 15, 7 20's, 2 29's, 4 30's, and a 40 breeder. I also have a lot of rubbermaid containers between 5 and 20 gal in size, so I'm not really sure <grin>.
It is really hard to say which tank is more "common". I guess if you stick to that one word (common), I would say a 10-gallon is. But that is only because you hear about them more. Newbies who are in need of help often talk about the troubles with their 10-gallon setups. After that, I'd say a 55-gallon is next in line.