My Bonzer is a good shape for head-high to DOH days, but I am currently stuck with it as an everyday board, so I've experimented a bit with the fins.
I started off with the stock 6-1/2" center fin at the recommended position (per GWS' post above). Moved it up a couple times, trying for a slightly more pivotal feel. Still wasn't getting quite what I wanted, but since I have the Red-X side fins, I was able to adjust those too. Bringing them all back slightly tightened up the fin cluster and the board came alive. In anything from fast shoulder-high waves, up to around DOH, it felt great. Dabbling with trying to free it up a little more in the smaller stuff, I've been trying a 6" cutaway fin in the center for the last few weeks. It feels OK, but if there's any juice, the loss of drive is very noticable (I accidentally left it in for a nasty day in Newport a couple weeks back with a bunch of BBers, and I was suffering on a few. Really guys....that's why I didn't go on those bomb sets! <img src="/forum/images/graemlins/tongue.gif" alt="" />). I'd like to try a 6" standard fin, or maybe a flex fin next.
For those considering new Bonzers, I can't say enough about removable fin systems such as Red-X or Lokbox. If I hadn't been able to adjust those side fins a bit, I'm not sure I'd still have the board. Instead, it's become my all-time favorite board in the wave sizes it was designed for. (now I just need a true small-wave board!)
edit: Maz - sorry, but I don't know the base & rake dims of the original center fin, but it's considered a "stock" Bonzer fin, per Malcolm's recommendations.