IMHO, the above post has a lot of poor and confusing information in it. Our system was supplied and installed by STI Solar Technology using 1/2 inch TuboPlus green pipe, it is a thermally fused copolymer pipe, made in Mexico by RotoPlas under license from the original German inventors, and is made with double walled construction, it is also UV proof, rated to 190 F, pretty much indestructible. Have no idea why it is not used in US & C, as it is great stuff.
1/2 inch is better for most homes because the volume in a 3/4 inch pipe of any distance means a much longer wait time for first hot water, unless you are using an on-demand down below, but even then, with decent pressure, 1/2 inch is more than sufficient for almost all residential needs.