noake,
The two biggest factors in determining voltage drop are the length and load. So you really need to determine both to calculate wire size.Maybe as an option you could bury a 3/4" pvc while you have the trencher and pull it in later. If its really long go up to 1" or even 1 1/4".

If you find out pump size and length I'd be glad to help you calculate your wire size.

"If you aim at nothing you'll hit it every time"

Zig Ziglar