Hmmm ... 3 x 5 instead of 4 x 6, not sure I'd want to go that small. 4 x 6 gives possibility for 4 x 4, too. If ya cut down a 4 x 6 into 3 2 x 4s, then the size of the board can be changed as needed.
I got one of those old cutout buildings that I see on the left hand side of the picture above, plus I see a few extra wooden beams on the front of your homemade scenery piece, too.