I Just made mine 4ft high, 4ft long and 2ft deep. 2 floors and a ladder for them.
If i was you ide give them litter trays, they'll soon get used to using them saves a lot of work cleaning out. and put vinly on the floors.
Put one up stairs one down and one outside(make sure you put a roof over it though or rain will mak a mess of cat litter or whatever you use. Also but a bit of beading down to hold them in place.
If you have the room, make a run 6ft long and 4ft wide, plenty of room for them to play then.
Ill stick some pick of mine up later, although the run is a little worse for ware, g