Its not just me. Most Set Designers I know walk into their set for the first time and feel "Oooh, is it too small?"
Why does this always happen? A well seasoned Designer once taught me to always add an extra 4'-0" to the overall length and width of an average room. (I'm not talking about closet/small space sets: those are always shot with wild walls for shooting)
The reason is the Unit. The Unit will eat up at least 16 sq ft of room space. Even though you have accounted for the Unit on the stage, the crew on set will still need the space. The camera alone eats up at least 8 sq ft.
Its true. I never feel like a room is "too small" in a house. I only ever question the size of a set, because I have experienced an unhappy crew, I have seen how they shoot, the space they use. Its a rule that has served me well.
And no one has ever said to me afterwards, "Oooh, I think the set was too big."