I guess the first thing to ask is how you intend to use the pool.

Got a load of kids? Go Bigger.

Just want to sit in the pool with your significant other and sip on an adult beverage, you might be o.k. with 18'.

I have had 18' pools (yes plural) and went to a 24 foot round. Go as big as you can if you intend to do anything more than dangle your feet.

Hope this helps.