Singe, twin, tri and Quad all have their respective character, but within each concept the rest of the shape matter a lot too. As you have noticed, with twin fins it is very, very easy to initiate the turn. This hold for pretty much all twin fin boards. The "up-size" thing you mention is a bit dependent on the shape though and mostly applies to narrow tail boards that often also have a very doomed deck. In my opinion, they use their volume rather ineffectively and is for example not very stable in low winds relative their volume. So in the end you don't gain anything by being able to use more volume. It's more a style thing which kind of shape you prefer. I would say that with the 2009 Starboard twin fins, you pretty much went with the same size as usual.
With the Starboard Quads I would also say you size them like you would size a normal bord. They have VERY big ranges, so it is a little bit less critical though. The Starboard Quads have much of that feel of easy turn initiation from the twin fins, but thay also have a lot more drive and acceleration in the turns, similar to tri fins. So you can say that they come in between a tri fin and a twin fin in feel - an excellent compromise if you ask me. Some things are just plain better like with the Starboard Quads than with twin fin boards and upwind ability is one such thing. Very practical in the kind of onshore conditions you describe. The acceleration in turn can be used to simply go faster and hit the lip harder, and you also have an amazing grip in the top turns whch mean you can pull quite a bit of spray:
and also follow your turns though better and turn tight and still come out with some speed.
Twin fins have a "looser" top turn and easily slide out. This can be an asset but also a bit of a problem, ie you can use it to your advantage, but particularly in cross on conditions when you are powered up, it can be hard to avoid a slide. In bottom turns twin fins rely more on a kind of soft controlled feel coupled with that "instant reaction". When you push a Quad, it accelerates through the turn, which means you have to be a bit more "on it". With a twin you can just "ride along" and then still very quickly redirect the board. Personally I've changed to almost only riding Quads since I think that combination of drive and looseness adds a lot to my sailing, particularly in "bad" conditions. I like twins too though, but the only case I can push myself to use them nowadays is in SUPER windy stuff. Yesterday we had powered up 3.5 sailing and I used my Quad 66. But then it got even windier, and for the last hour it was over 40 knots. Then I though the softer nature of the twin was nice (in this case a smaller version of mast years Evil Twin 70). In this example, you can see I'm just coasting along with no bottom turn drive, but yet can quickly flip the board very and hit the lip. This is one sort of thing that is easier on a twin. But in more normal situations I get "more turns per hour" on my Quads.
(images from Christian Johansson)
Regarding small or big in front, all I can say is that I think the solution Starboard have choosen works very well.