Want to put electric in a pool cabana.have a distance of about 100 feet.was going to put a 100 amp sub-panel.what is wire size for that distance?

#2 THHN would be slightly more than really required but I'd opt for that over #3

I'd use #4 and put it on an 80 or 90A breaker. Unless you have electric heat out there 100A is way overkill.