He kinda described his motor and he's asking for roll-out so rear tire size doesn't really matter as it's kinda included in same.

I have good luck starting around 44-45mm rollout with some pretty good 3300's. You may be able to go some higher on your 3800's but you may find that your lap times begin to fall off due to lugging your motor before you find you start to dump. I'd start with 45mm or so then work my way up .5mm at a time. Take the average of your 5 best laps for the heat and see if it's going down or going up.

Scottrik