Same size as on the 26er or a size larger/smaller?

In my head I'm thinking the wheel is a larger lever so I need more torque from the disk brake to apply the same force at the ground?

Bigger looks better on a big wheel but actually it don't make no difference – the brake is just there to convert kinetic energy into heat.

tinas – bigger will be better for the reasons you suggest, whether it is necessary is another matter 🙂

Given the wheel is 10% bigger ALL COMPONENTS must also be 10% bigger or it won't work.

I run 180f 160r – works for me. But I'm a big chap.

I take a larger stick to jam in the front wheel

205mm disc actually

203 front and rear here, not sure there is a great deal of difference over 180s but I had them so I used them…

160 front and rear here.

180 / 180 here

The bike weighs about the same and you weigh about the same, so the same mass is moving so you'll need about the same brakes to stop.

Agree with halfbee size of breaking power can only relate to the weight your trying to stop and the speed your going at. I suspect you'll see bigger rotors on 29'' because your larger gentleman will favour the big wheels.

I'm demo'ing a 29'' weekend after next, can't blooming wait!