Hi Nick,
I bought a gennaker for my 460 about 3 years ago. Admittedly I have a bowsprit with a D ring so didn’t need to worry about that. I didn’t go for the max size because I didn’t want it to be overwhelming so went for a “mid-size” sail. I guess to make an easier comparison the gennaker sail area is 1.36x my “main plus genoa” sail area (105% or 108% genoa). Snuffer makes life very easy if you’re used to spinnakers . . . .
The sail makers will recommend size if you give them your dimensions – but to calculate that you’ll need to identify your tack point.
Hope that helps. Tom