Use whatever size box is best for the intended occupant. For Gwen’s cabin I used the box a carseat was shipped in, but I didn’t take pictures of its construction so I made a scale model.
All the techniques I used can be applied to bigger boxes (or smaller ones if you need to house some mice).
Fold the taped-together flaps over the top of the cabin so that they overlap the lower half of the roof. The gap between the two pieces of cardboard allows it to fold easily and form the ridge. Tape the sides of the roof piece to the gables. For the big cabin I used the cut off short flaps to make a third roof piece but it was purely for aesthetic reasons (I wanted each roof tier to be the same size) and isn’t necessary.
Now that you’ve built your cabin, you can decorate it! Im an idiot so I used an ultra-fine sharpie to draw woodgrain all over Gwen’s, which took a million years and is basically invisible, so maybe don’t do that. But you could draw a less pointless pattern with markers, paint it, or cover it with paper.
I tried to lure Gwen into the cabin for pictures by staging it all nice with fur rugs and pillows and toys but NO DICE. She still doesn’t care to go inside and it doesn’t help that she has a cold today that is making her very grumpy. Someday though. SOMEDAY.