depends on the bike and how much power it makes. For older bikes, 160 seemed to be the set standard, but most 600 bikes back then only made like 75-80 rwhp. Now days, modern 600 bikes producing 100+rwhp use 180's to increase the grip.
For tires, when your going straight your U shape turns more into |_|, where the center becomes flatter - more surface area. When you lean, the same principle applies where the tire flattens out to increase traction/grip. 160 tires have a much narrower center flat spot, while 180's while, they do have similar curved ends on the sidewalls, are bigger in the middle. 190's and up are mostly thicker in the middle too.
Think of the 160 being (_) , easier cornering and you can flick on a dime (depending on bike).
180's being (__) , easy cornering but still enough center surfance area to move.
190+ (___) . When you lean and roll the tires to the sidewalls, there esstially will be less amount of tire when cornering. And they prove to be a bigger challenge when trying to flick the bike from side to side.