I have wired an exterior garage with some very basic electrical. I have 6 outlets (for convenience, only one or two would be used at once), Three 75 watt lights and an exterior spotlight.

The garage is about 400 feet from the breaker panel. What size wire would this require (wire will be buried) and is 400 feet too long?

Assuming a full 20 amp load for one circuit, # 4 copper will give about 5 volts dropped, the max is 5% of 120+6 max Vd.

#2 aluminum will give 5.1 Vd, so is a good substitute, if allowed in your area.

Would be the same for two circuits.

Use XHHW insulation if the entire run is conduit, or use USE or URD cable if direct burial. Even use 4" DB duct if direct burial cable is used, works good as extra protection, and allows upgrades.