The tops of the doors and windows should come just above a based figure's eyes and be proportionally thin. By adding a threshold block at the base of the door, the door height can remain the same, but the door looks even smaller. This makes the small building appear even roomier. For the same reason, use the thinnest bases you can on your figures. That way you can reduce the height of the buildings without having the figures appear obviously too tall for them.
Stairs are an exception.
If you want figures to be able to stand on the stairs, make the steps unrealistically large, over half the width of your standard figure's base. To the Major General, at least, a slightly toylike look to buildings is not objectionable. There is nothing wrong with keeping that childlike sense of enjoyment alive. If your gaming style accepts the convention of soldiers standing on the roofs of buildings, then think about your building sizes in terms of your figures' base sizes. Sometimes increasing a building's size only slightly will allow an extra row of soldiers to get onto the roof.