You didn't give a lot of detail about the board, but the glass job sounds like it's a more of an old school log type board... big and heavy at least.
Just to add to the conversation... you really don't need to worry about drag on single fins. A well foiled center fin is very efficient. Bigger fins do have increased drag, but also increased drive and directional stability. So drag typically is not the issue with an over finned board. Responsiveness is. It's a trade-off... drive/trim speed and hold vs. responsiveness and quick turns. I'm not a fin guru, but at least that's been my experience. Others might be able to give you better feedback.
That being said, everybody has their preferences.... but I'd say, regarding size, 9.5 is the absolute minimum... as Matt and others have said, 10" might be better. I wouldn't go getting a flex fin to start with on that beast. I'd advise sticking to a California Classic type fin. I think the Greenough fin will be too flexy. If you really wanted to go with a flex fin, go with the Ferberow... a bit more width, and a bit more even flex pattern from base to tip.