I guess there is no 'THE' right size. I've seen as little as 1 square foot recommended for a pair. There is an official care instruction issued by the government in Denmark where I live, and that says at least 39x39x19 inches. Some people say anything smaller than a greenhouse or a walk in aviary is too small.
I prefer to keep mine either in aviaries or in large cages that are 47x23x23 inches. That gives me room to place fake plants, dust baths, shelves to hide on (if one is aggressive the shelf seems to be the one place the other can relax) and under, tunnels and such, while still allowing them room to move around. The height of 23 inches is just enough for them to stretch their wings a little. I pad the top of the cage to prevent boinking injuries. The length of the cage means I can clean it without causing the birds to panic - they just go to the other end and stay there unless they become too curious about what I'm doing. When I move on to cleaning 'their' end, they go to my end.
My buttons go broody very often in such cages. If you don't wan't a male, I guess broodiness isn't something you are particularly aiming for, but I believe it means they thrive and feel safe in the cages.
Two females with each other shouldn't cause problems. It's true that they tend to get territorial if there is a male with them - I've had to give up on keeping trios several times, though I do have a trio in an aviary which coexists nicely. The same trio didn't work in a 59x23x23 cage, so it seems to be a space issue when they can't live together.
Make sure you introduce the females to whatever you intend to keep them in, at the same time. Even if they come from different places, it shouldn't take them more than a few hours to accept each other if you do it like that. If they insist on pecking/chasing each other, separate them by a single layer of wire until they stop attacking each other through the wire, then try again.
