All dimensions are width by length (Horizontal, Vertical).
Note that this doesn't include the area in front of the building where you cannot place items (if you want to include the area where you cannot place items, increase Vertical by 1 value):
BUILDINGS:
Player - 5 by 4.
Villager - 4 by 3.
Campsite - 4 by 3.
Able Sisters - 5 by 4.
Nook's Cranny - 7 by 4.
Museum - 7 by 4.
INCLINES:
Incline - 4 by 2 (Horizontal Placement) / 2 by 4 (Vertical Placement).
BRIDGES:
Bridges are a little different, and I'm not entirely sure if these value are correct. But, bridges are 2 wide (horizontal and vertical) and can stretch as far as 6 (or 5) and be as short as 3 in length. As for diagonal placement, I have no clue.