You never need to drag to an exact size.
- A wall: select it and type its Length. Its far end moves; walls joined there follow.
- A room: select it and type Width and Depth. The walls around it move.
- Furniture: select it and type width, depth, height and Rotation. Reset to catalog size puts it back.
Fields understand 12' 6", 12.5 ft, 150" and 3.8 m, whichever units the plan is in.