RT is awesome because it's small and that should be a consideration for the gates. Make sure they are packable/foldable/easy. Gates with guy-lines don't work well because some race venues are on concrete. Secure with stakes or sand-bags, no guy lines.
I know you are asking about size but I have to throw in some words about color choice. When you are choosing your colors, put yourself in the goggles because some colors really don't work well. No green, no orange, tan, or earth colors because those become invisible through the fpv camera.
We have a selection of gates at our disposal and the white (or mix of) seems to be the favorite.
0.75 - 1.0m - MICRO: 4' soccer gate, hula hoop, other gates?
If it's for Inductrix style racing, 4' is way too large. We typically use 2' foam core.
1.5 - 2.0m - 250: 5' X 5' gate, 6' soccer gate
That's about right. We don't have a lot of room, unlike the huge fields on the east coast, so we use gates similar to:
3.0m - wing race