The Nimbus 29" unicrown frame is designed to fit the Big Apple tyre (29x2.35"). The biggest 29" tyre I know of is the WTB ExiWolf, also 29x2.35".
Thanks Spencer that’s what I want to hear:) Maybe knee-whacking is something that declines as the uni is used more?
As for the Weirwolf, it’s a 29x2.55! I only know about it from reading this thread the other day. It’s not listed on the WTB site at all, but at least one shop in the US claims to be selling them.
Well, I’ve just taken some measurements, and I think a 2.55" tyre will fit the Nimbus 2 frame, as long as a) the wheel is 100% true, and b) there is no mud on the tyre.
So, ummm… it’ll be ok if you don’t want to use it off-road 
STM
Oh bugger. Still, the Weirwolf might be a bit smaller than advertised, and also the extra tyre clearance could come in handy for using a normal tyre in the mud.
I think my Nimbus 26x3 muni is probably built with a Nimbus II 29" frame - there’s an enormous amount of clearance between the tyre and the crown. I’ll measure it if it helps. I’ve never had any trouble with hitting knees on mine like Tony mentions - and mine’s one of the kind with the really wide tops (tubes are only oval at the bottom, round at the top, with plastic plugs rather than welded on caps). I’m quite tall though and I very rarely hop, so perhaps that helps.
Rob