I can't advise regarding the maximum size, Andy, but we have had two replacement jockey wheels on our Triton and both were 250mm. I wouldn't want to go any larger than this due to fouling the bump plate on the hitch when manoeuvring.

The first, a tubeless pneumatic came with a new Alko jockey wheel assembly and was excellent with our mover until the rubber started to perish after only a couple of years and air began to leak through the tread. I replaced this with what Alko bizarrely refer to as a 'soft wheel' which is in fact solid and hard.

This thread may be of interest: Jockey wheels