For mod practice I start tires at 42 to 42.5 mm (huge!!) and they will normally last for at least six to eight full 8-minute runs of mod before they are too small. I personally find that going any larger in diameter just makes the car drive like shit since the roll centers are jacked, so I don't even bother.

For big races (medium to high grip) I would start with 41.5 mm and then put a practice run or two on them so they are right around 41 mm for quals and mains.

EDIT - sorry, realized I didn't explicitly say I was referring to rears. For fronts just take off one mm or so from the above, depending on how much rake/stagger you like running.