Wow, I seem to have started a bit of a controversy. Not going to reply to everyone individually, but I'll add a few more thoughts.
I'm glad I'm not the only one who thinks a 33" (84cm) low-volume board is hard to waterstart, at least a few people here agree with me. I've also seen a guy here locally, who is a talented foiler, but riding a *very* small custom board (probably something like 27"/68cm) struggle repeatedly to get the board up out of the water and start in any lightish winds, when other less-talented foilers can easily waterstart on more "standard" pocket boards like the smaller dwarfcraft (42"/107cm). The dwarfcraft is not only a bit longer, but also has more volume, and I think both of these work together, as a thicker, more buoyant board, even when shorter, will still float your feet on top of the water and avoid having to drag yourself up to the surface and out of the water so much, making the start easier in lighter winds. However, the "short but high volume" board is not a popular style at all. Someone on here rides them (Horst Sergio, maybe?) but aside from his custom boards, nobody else seems to like short, high-volume boards.
I rode Jim Stringfellow's custom 39"/99cm low-volume board yesterday, and while it exhibited just a touch of the same "board has to start from underwater" tendency as the 33" KS I rode before, it was *much* less. This implies to me that at least for my stance/body position/riding style, a low-volume board starts to get too small to water start easily around 1m in length. A high-volume design could probably go a bit shorter than that, but like I said, there aren't a lot of high-volume boards shorter than 1m out there.
But the thing is not so much that I'd go smaller except for the difficult waterstart, it's that I don't think there's much advantage to going smaller. When I swapped boards with Jim Stringfellow yesterday to try his board, he rode my carbon T38 and his thoughts were "the extra length didn't bother me". The T38 is 51"/129cm. Yes, in a very technical sense, a shorter board has less swing weight, but the difference between a 2.3kg 129cm board and a 1.9kg 100cm board is just not that big. An extra 400g hanging just a few cm in front of your front foot does not make a huge difference.
The whole conversation reminds me of the discussion around pointed noses on performance surfboards. A surfboard with a blunt nose is safer, less damage-prone (the nose is the easiest part to break), easier to travel with (it can fit in a smaller bag), and has (same as this discussion) less wing weight. But they're never caught on in 30 years in any large numbers. People keeping building and selling and buying and riding boards with pointy noses, and there's no real justification other than they "look fast" or "look like a performance shortboard should look". It seems to persist mainly out of fashion.
Freeride hydrofoiling is still pretty new, and as people like Greg Drexler and Fred Hope have ridden smaller boards with good success, people have wanted to emulate them and gone shorter and shorter, which in my experience offers questionable benefits below something like 1m length and 15l volume (depending on preference and weight). But it seems to me like it's been pushed past the point of diminishing returns (see that guy on his ~27" board that can hardly get moving on it as an example) even though people seem to like in entirely unquantifiable ways (see several comments in this thread about the "feel" of small boards), or saying "they turn faster" which sis probavblyh strictly true, but I bet in almost immeasurably small amounts. I haven't seen any maneuvers that talented foilers can accomplish on a 90cm board that they can't also do on a 110cm board, because I don't think the actual performance difference is really there, the difference is just too small. I definitely do think the current perception is that smaller boards are "cooler" lately, among freeride hydrofoilers, and when I say that this is mostly "fashion", this is what I mean. People are riding tiny boards because Greg Drexler is riding tiny boards, not because they can complete their tacks or switch feet (even though someone in this thread did claim that) or surf waves better on a smaller board.
If you really want to make noticeable difference in the maneuverability of your foil, switch to a smaller stabilizer, it makes 20x more difference in making the board feel looser and more maneuverable than cutting 15cm and 300g of the board.