There is a file in each cars data.acd called tyres.ini. Adjust the width, radius, and rim radius. The measurements are in meters, so 235/60/15 would technically be .235 .3315 .1905.
However, there seems to be an issue with doing straight up measurements like that.
Assetto corsa devs have said to add 1 inch to the rim radius or something. Something to do with rim/tire overlap where the tire bead hooks in to the rim, i believe. Content manager seems to calculate the figures slightly different as well. I've been playing with this lately, and found that to get CM to report the correct values, you need to add .5 inch to the rim and then another .5 inch to the total radius. Don't forget to convert to meters. .5 inch is .0127 meters. So, the correct figures for my example tire size would be:
Now CM reports 235/60/R15
The only other issue is that if you change tire size on a car, you haven't actually changed what you see on the screen, only the physics. You may find that if you make the tires with a bigger radius, the car will appear to be riding above the surface on air!
If you play around with CM's Tyres Machine Editor after you get the size you want, you can make some new tire sets that actually change the physics. Just click "replace tires", then "generate new ones", then scroll down to "+create new". Takes a while to figure out how it works. If you google it, you will find some rough explanations. Once you have a new set made, apply them to your car. Fun stuff!