Hi Daz and welcome,
I think that the standard wheels that Richard supplies are around 15 inches in diameter, with 8 inch centers.
I have used 8 inch trailer wheels with 4:00 x 8 tyres, which are a little larger at 16 ins dia.
Others have used 6 ins quad bike wheels (Quadra, Neil, Gremlin) to great effect.
It depends what type of vehicle you're building really!
So, you pays yer money, you takes yer pick, so to speak.
Hope this has helped.
Regards,
Mark.