Just got a rear tire from a guy and it's for a ninja 400. it's smaller than my OEM size race tires I have now. So did some research and they say racers play with the sizes... So just wanted to get another point of view of wts the best size for racing?
Know that the # on the sidewall is a rarely the actual measurement...then there's shape. Hopefully some guys who run varying sizes will chime in. My old 400 is 110/140, but I've run other sizes on the back, and it is...fine.
You may enjoy getting specs from various sites and making a spreadsheet. I did.
If you don't get answers from folks who actually run that bike, I'd also check manufacturer sites. I know that Dunlop lets you enter your bike and it spits out some desirable/recommended tires. Wider tires may provide more grip but will likely increase steering effort. There are a lot of variables so folks who've run various options will have the best data.
Unemcumbered by any facts, I suspect that a change one size in any direction won't be terrible. Some of it you can measure, like width, by looking at the bike and measuring the space available (for instance, a 190 will not fit on my VFR, but a 180 is perfect). This would be fit only, not whether it would feel good to ride on.
A lot of owners put 120/160 on my old 400 model. They loved the look of it, but it handled like crap with that fat back tire, and the fat front one was squeezed on the narrow rim changing its desired shape, so it provided bad feel and less grip.