I have a 230 volt spa that runs off a 50 amp single phase breaker. The length of wire I need to run is about 135 feet. What wire should I use?

What do the instructions say? Probably #6cu.

Can you elaborate a bit on this installation?
Inside?
Outside?

Do you know ALL the important codes related to wiring a spa??

The instructions say "at least #10 AWG" and the wire should be "four-wire electrical service (two line voltages, one neutral, one ground).

The spa is outside, but on the other side of the house from the panel. I was going to go up through the atic and back down.

The codes for spa are VERY specific and VERY important.
I suggest getting hold of a few good books to help you.

Sorry, but IMO this is really NOT a DIY project.