Btw, what's your approx. room/floor height for these buildings?
My building methods couldn't be less scientific

The size of my doors are 4cm x 2cm. That may seem a bit high but with the size of figures these days it works for me. Anyway, I use the door as the point of reference for the rest of the build so when I calculate the dimensions of my build it is in relation to the size of the door. Sometimes I use actual buildings for inspiration so I'll measure the picture of the building assuming the door is 4cm x 2cm and scale it from there. Clear as mud?

Saying that, obviously some doors are larger or smaller depending on the type of building but that's easy to get around.
The dimensions of the shop front in this build are 17.5cm x 9.5cm. If there's any other measurements you need feel free to ask.