Pete,
I have been using a small one( about 9" in diameter and 36" long) for diving for years. Helps the dive boat find you. Easy to inflate since it takes a lot less air and easy to wave around to get someones attention. The problem with the large tube is getting it inflated. Not a problem with a tank of air atttached to your back. Not so easy in large waves with wind and you are spending most of your energy trying to just keep from getting a mouth full of water. Now if you plan on using it as a raft you might want the large one.
And I don't understand the diameter issue. The idea is the length so you can get it up high for soemone to see.
Orphan.