about to get some wave fins for thruster setup for 105 freestylewave board and would appreciate some advice on what sizes to get with this setup

thinking of maui ultra xtri 12.5 slot box and 20 power box centre fin for smallish waves??

Sounds about right for an "average" weight person say 80kg. If you are 90kg+ then you may want to go slightly larger say 13s + 21. The MUF X fins are awesome - quite powerful so don't need to go too big.