There are actually several different sizes for all 3 tables but for the most part Billiard and Snooker Tables stick to one size for tournament play(at least in the U.S.A.), while games like 9-Ball and 8-Ball have pool tournaments held on a variety of sizes. This is really due to the size table the room hosting the tournament decided to buy.
A Standard Snooker Table is 11 ft 8.5 in by 5 ft 10 in but usually referred to as 12 x 6 Snooker Table.
There are Smaller 9 ft 5 in by 4 ft 10 in or 5 x 10 Snooker tables that are in a few rooms but definitely not considered the standard.
Pocket Billiard tables or Pool Tables come in 3 different sizes,
On all of these tables the playing surface is rectangular with a 2:1 ratio.
3.5 ft by 7 ft – Usually referred to as a bar table or bar box. These tables are often home to 8 Ball Championship Tournaments.
4 ft by 8 ft – Probably the least common size
4.5 ft by 9 ft – Standard size Pool Table for Pro tournaments.
Once in a while you may run into an odd size table like a 5 x 10 Pocket Billiards table but I doubt you will run into one in tournament play.
3 Cushion Billiards & Carom Tables are regulation sized at 5′ by 10′ for match play. Although I am sure there are exceptions I have never seen any other size for these tables. Once in a while you will come across a Billiard Room that has heated slate Billiard Tables to ensure the table plays consistently no matter what the Pool Room enviroment is.