. . . membership updated.

Dave is right, you should have an electrician make the installation.

So far as breaker size -- install the largest breaker appropriate for the wire size and run length. I believe with a 50' run you can install a 30A breaker with #10 wire, and a 20A breaker with #12.

Let me caution you again: if you don't get the wiring and bonding right, you may KILL someone. Wet, barefoot people eventually touch electrical things around a pool: make it fool-proof safe, or kill somebody: your choice.

And a second caution: years, and years ago, I was indirectly responsible for nearly killing a lifeguard, because I assumed that GFCI breakers and receptacles 'failed safe' -- that is, that when they failed, they failed OFF. That is not necessarily correct. My setup met codes (at the time), but failed DANGEROUS and a guard got a severe shock as a result. (Yes, he was doing something stupid, but sooner or later, someone around YOUR pool will also do something stupid.) Do NOT depend on a GFCI as your only safety: bond and ground as well!

. . . membership upgraded.