Replacing Main Bearings

I want to put main bearings in my 4.0 89xj. It has 160k miles. How can I determine size? Undersize, oversize, original? Bearings seem to come in sizes .040 .020 etc

Oversized bearings are for reconditioned cranks. You remove the crank and have the bearing journals turned down to by a certain amount to remove all the damage to the journals and give you a fresh smooth surface.

You shouldn't really get oversized bearings unless you've had the crank cut down. You'd have to pull the crank and measure the crank journals to determine if they need oversized bearings anyway.