Go with the standard rootstock; at maturity your tree will be no more than 3 meters high by 3 meters in diameter; you can prune it to be smaller, or to fit its space. If you can find it, for your climate I might choose rootstock Volk Americana or C-35; after that perhaps sour orange; I would avoid Carrizo as it is quite susceptible to phytophera.

Based upon a Durling Semi-Dwarf I planted 5 years ago at a neighbors that is 3' tall and 5' wide, I agree that you need a Standard. I planted two Durling Standards last year against a wall that I will espalier to cover the wall. There weren't any branches below about 20" when planted but a couple came out since then so I will have no trouble getting branches 9-10" off the ground for the bottom horizontal branches. I believe Durling uses C-35 for their Standards. If you are near an Armstrong Garden, they can order you a Durling Standard. Most places will have only Semi-Dwarf trees.

Thanks for the replies, it's great to get real world experience from people on these forums.