I'm curious about this length business......
I've seen trunks which seem to be pretty standard and are probably what are all thinking of, whereby, there's a bit of hypalon glued into a tube shape which has to be kept up to avoid water entering the boat.
My trunks are flat (two pieces of pvc heat welded together), and are quite short (300mm ish). When the boat is at rest, they sit under water, but, because they are flat, water does not enter. However, when the boat is going on, they are not in the water, and so any water on deck drains out of them. This works really well, so why donít' more people use them?!