3 inch tires
You can get the chain to clear fat tires by removing the drive sprocket on the motor, flipping it around, and adding a washer or two to space it out away from the motor. Make sure to use red loctite when you tighten the drive sprocket back on. These two things will move your chain out just barely enough to clear the tire.
Now may need to clear the inside of the frame. Add an extra nut to the rear axle on the inside of the frame on the motor chain side to push the frame apart a little, and you should be ready to go. This is how I motorized my Nirve Switchblade Chopper and got to keep the original fat tire on the back.
The chains might be pretty close to the tire on each side, so make sure you adjust the wheel position with the bike straight up and down, not leaning to the side on it's kickstand. If the wheel is rubbing on one of the chains, you can just point it left or right a little. It's not going to be noticeable if the tire isn't perfectly aligned and it dog-tracks just a little. If you want to get super cool, you could "dish" the rim by tightening the spokes on just one side or the other, which would offset the wheel to the side of the tightened spokes while keeping the wheel perfectly in line with the frame. I wouldn't bother with dishing it myself, but it is an option.
If you're still having a hard time getting both chains to clear the wheel, add a washer or two to the crank on the inside of the pedal-drive sprocket. This will push the pedal-drive chain out just a bit and it will give you more room to adjust the tire away from the motor-drive chain if you need to.