Generally accepted consensus among european competitors is 5 boatlengths MINIMUM. Any smaller than that and I'm certain the RC would get a raft of complaints. Although I don't think I've ever seen it written down anywhere?

In larger fleets (and often smaller ones) the gate is usually much larger. IMO faster boats such as cats need larger gates because of the distance travelled in short periods of time. The RC needs to allow competitors sufficient room to respond to each other and therefore I'd want to see cat fleets with larger gates than the dinghies of similar size use.