"Whatever size is intended for the bike is the size which should be fit again, for many reasons. Certain-size tires fit on certain-size rims, so you need to replace the correct-size tire onto the wheel you have. The rake and trail of the bike and front/rear weight bias are affected by the front and rear ride height, so you need tires with the same circumference. Anything else can have very negative effects on handling or grip. If you try to put a wider tire onto the same rim, either for looks or performance, you are pulling that tire into a narrower wheel than it was meant for, and you often lose footprint because the tire profile is now wrong. Plus, the wrong size tire can rub against the motorcycle and get damaged during use.