I think 12" blocks are the most popular because of the possibilities of units that are divisible by 1-1/2, 2", 3", 4", 6"....no 1/4", 1/8", 1/16" measurements to worry about that you get when you start to do smaller blocks.
The smaller blocks are so cute...and they can be addicting...they take just as long to make as the bigger blocks.
There are so many patterns to chose from in all block sizes...personally, I like 8"-9"-10" blocks the best...12" blocks start to be too big to play with as much...can't get as many in a quilt.