What size wire do I need to run 200 amp power from my 200 amp power pole to my barn? I intend on using the breaker box at the barn for water well power and for operating tools from my barn. The power pole is about 350' to 400' away from the barn.

At full capacity of 80% of 200 Amps, you would need a #600 MCM aluminum to keep the voltage drop below 3%.

The actual amp load is a critical piece of information when doing voltage drop calculations.

If you can determine what the max amp draw would actually be, perhaps you can do with a smaller wire size.

Be more accurate on the distance also.

Just for clarification, are you running an entirely new service to the barn (with its own meter and a separate bill from your electric company), or is this powering a sub-panel in the barn? If it's new service, is the meter at the pole or at the barn? Also, are you burying the wire or running it overhead from the pole?