What size rear
 

I've resorted to Road Smarts to get some decent wear out of tyres on the XT and they work well and use a 160 on the rear,I have a bit to do with bike trade and a 170 and turned up at the right money aside from the fact I wouldn't expect it to turn as well,but does anyone use a 170 on the rear.

I have used a 170 on mine and it was fine.

I presume it cleared the chain guard then ?

Yep, there were no issues with clearance. I bought the bike from a Yam main dealer with it on. Only changed it because I bought a pair of new tyres.