housing size

You want it taller and at least 1 to 2 grids longer. They need to be able to hop and stretch out.

The latter [5x2x3] sounds fine to me for the time they'd be in there. But I would have it with a 4x2 upper story and that's it. You want to give them room to stand up. Also, I've found that instead of ramps [if this is what you plan to use], my bunnies like milk crates turned upside-down with a piece of cardboard and a towel/carpet on it to nap on and to use like a stair.