I'd use PVC, at least 1" and maybe even 1-1/2" to avoid flow loss due to friction. You must remember that there is no way to positively guarantee the pipe will not freeze even if it is buried below the frost line. Therefore, I'd suggest you put a shut off valve were the line branches from the main supply and install a drain valve at the lowest point. If you have an air compressor, you can blow the line in the fall much like we blow out irrigation lines. You might go forever and not freeze up, but on the other hand, this could be the 100 year winter, and if you design it to be drained, you won't have to go so deep.