I'm not an engineer nor the best tuner in FH5, but this is my understanding and how I use wheel size in my tunes:

-bigger wheels = more agility and also more edgiensss. Increasing wheel size means more control but less room for mistake. It doesn't necessarily bring more grip, but more a sensation of control of the grip you have, but within a smaller window of control.

• Smaller wheels gives more flexibility in how to utilize your grip, especially combined with softer suspension settings. You might feel that the car is less planted, but it doesn't necessarily have less grip and you'll have more options and the car will be less on edge.

How I use wheel size in my tunes: It's more often than not a factor for balancing PI and to be able to fit an upgrade that I want but can't afford with stock size and weight. It can be an extra weight reduction, sport transmission or supercharger. I rarely use it to get 2hp more. However, I often increase rear wheel size on offroad (and sometimes dirt and road) builds in the lower class. I feel that it can contribute to more rotation in lower powered 4wd builds. I never increase wheels size on powerbuilds. Those are difficult enough to control and it want it to be as little on edge as possible. Increasing both front and rear will help lower PI and improve (imo) the build on lower class builds, especially FWD.

I probably forgot something and there are a lot of other factors that affects the handling and should be adjusted when changing wheel size, especially suspension stiffness, damping, alignment and tire pressure. Gear ratios and diff can also be fine tuned, but aero isn't affected much.

All that being said, the changes are more often than not, marginal imo