I'd be happy with around 300 or 400cc, unless you are Jonah Lomu size...
Sako75's point about centre of gravity is really good. There is a larger quad at the conservation sanctuary, it is harder to get around tight spaces, and a nightmare on the very steep downhill sections, wants to do ass-over-end.
Guy next door sells 2nd-hand quads. Once you've worked out roughly what you're looking for, price range etc, lemme know and I'll give him the specs and see what he can source.
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