Instead of trimming, I just prepared for my larger tires by taking off all the plastic where the mudflap attaches. It leaves quite a bit of room and to be honest, I think I like the look of it better without the big plastic bulge that used to be there. (Otherwise, completely stock '03)
In case anyone else wants to remove their plastic peices, here's a little info on them as they can be kinda tricky. Obviously there is about 3 or 4 screws to remove on each side. Afterwards the plastic is only held on by 2 plastic clips.
The upper clip can be dislodged by prying towards the rear of the car and the lower clip by prying downwards. I just gave it a good whack with my hand and they popped off.
If you were to remove the white plastic holders, it wouldn't be very noticable at all. Perhaps one day I'll get fancy with some fiberglass and make a smooth cover that fits the area better. Replace the screws you took out so that it will hold the inner plastic wheel well parts intact and keep everything from falling apart
The final result - these are new maxxis mudders, the website says they are 31.8 tall and 10.8 wide (265/75/16) - this pic shows the amount of clearance in the front with the wheel turned to get as close as possible to any body material
I'd say you could probably go up another size in tire and not have too much rubbing problems from the rear, but the front part of the wheel well would start giving you rubbing problems while turning. A new front bumper would cure that