Thanks Pleiades. I was in touch with Talon, and seems that OTR setup is not an optimal solution for an adventure bike like xt660z. In fact this is LC4 hub (lc4 is 70 kg lighter then xt660z), and as you have said - 120/90R18 is the right size. Eventually I can use 130/80 (130/90 would be better, but most probably there will be problem with swingarm clearance), what limits dual tyries selection to Michellin T63.
The other option I have is to use 17x3,50 on the stock hub. Do you have any exeprience with such a setup? This way or another, I need new rims for next season :-(