I have a 20 x 40 vinyl IG and really like the size of it. I've had 15 + people in it and we still had room to play. I never heard anyone say they wish their pool was smaller!! Well maybe while they were manually vaccuming it...ha. When I have parties the adults tend to stay in the deep end and the kids in the shallow end until its time to do cannonballs .
I've had a person with a L-shape tell me they wished their pool was rectangle due to safety reasons. They said kids loose track where their at and swim into a corner and bump their heads. I guess they can do the same on ladders, steps, etc too.
Make sure you get a diving board and make the pool as deep as possible. I turned 40 this year and still have a blast diving, flipping and doing cannonballs off the board. The kid in me comes out pretty quick ... no wife to tell me I am going to kill myself....ha. Good luck.