Hi Strelnikov..(is this a "Nick" or your real name??)
welcome to the first russian Bulldog-rider to registrate here..
The standard size for the front (120/70 ZR17) is ok and you should´nt change it. For the rear you have a few options : either the 180/55 ZR 17 or 190/50 ZR 17. Both tyres fit the original rim without further modifications.
Depending on the tyre-make (Michelin, Continental or Metzeler for example) you might not have enough clearance between brake-arm (this metallic bar connecting the brake-caliper and the swingarm) and the tyre. Ride-In, a Yamaha-Dealer in Glauchau -
www.ride-in.de - who has lots of parts specially for the BT1100, has an easy and cheap fix for that...two longer screws/bolts and a spacer to relocate the bar to the outside..
Come to Austria for our International-Bulldog-Meeting from the 11th to the 14th of June 2009 and meet over 150 other Bulldog-riders from all over Europe..
cu there
Axman