@Jgb19830 Are you looking at knobblies or road tyres?
The main issue is the width of the rims, if you are using stock rims you are probably at the limit of tyre widths already. I had Avon TrailRider tyres on my DTR with F-90/90/21 & R-120/80/18 and they were wider than stock. You do not get much benefit from fitting wider tyres on narrow rims, as the tyres will form a 'V' shape and ultimately you do not get much extra width, plus a decrease in the contact area and therefore less traction. All_in_all the overall effect is a decrease in the road holding of the tyre.
Stock rim widths are F=1.6 & R=1.85. you will need at least a 2.15 rim for the rear, which will enable the 140 tyre to be fitted. I would suggest that you do not go any wider than the 140 as the chain will cut the tyre wall. If you go wider on the front the bigger tyre will throw out your speedo and give a false reading, due to the wider tyres having a larger overall diameter. See this handy Avon chart for recommended tyre and rim widths;
On DTX SM rims I run 120 front and 150 rear Conti Attack SM tyres, you don't really wanna go much wider than them on standard rims as the profile is pretty agressive, takes alot of lean to get full use of the curve but they feel very good on tight mountain roads.