There are several possibilities:
There will be a fight.:fight:
The bigger one will chow down on the smaller one (especially if smaller one is much smaller).:evil:
One will hide and may rarely been seen.
Possibly they can work it out if the size difference is minor.
Basically, females of similar size can be in the same enclosure but almost always there is a battle for dominance. When the females battle for dominance the injuries can be minor but occasionally major. I've had females live together for years but the first few days always included some butt kicking.
If two males, the smaller one is doomed. Don't put to males together. You may already know this.;-)