it seems counter-intuitive, but it's actually easier to pump the water through the larger pipe because it has less resistance to flow.
in this case, sizing up from 3/4 inch short run direct from well pipe is indicated due to the longer (more resistance) run from the well house to the barn a good distance away. that said, if the whole pipe is easily accessible, there is certainly nothing wrong with replacing the 3/4" pipe with larger pipe when he's got it broken to t into it. most well pipe coming out of the ground is 1.5 to 2" OD for normal residential service.
i would go with whatever is cheapest and readily available in my area in the 1-2" size. also, don't forget that elbows and other connectors get rapidly more expensive as the size increases.
amp