Size of center fin would depend a bit on length of board, but for a 9' board, the tri-fin set up I use is a seven inch HP center fin (I liked to push mine forward to within about 1.5 inches from the front of the fin box) and a pair of standard sized sidebites (not sure what size they are but the shop could let you know).  I move up to a 9" center fin if running it as a single.

If you need to know about what angle to set the sidebites, someone with more knowledge than me will need to chime in.

Hope it helps.