My understanding is that the room sizes displayed should be the internal measurements. However if you place a 3m x 3m room, it is clear (by reference to the grid) that the distance between the walls is less than 3m. If the internal face of the opposite walls are set to line up with the grid, then the room size is about 3.21m x 3.21m
Similarly, if you place a 3.0m line (with arrowheads at both ends) between the walls, the arrow tips are displayed (and printed) partly sticking into the walls, rather than just touching the walls. To make the line fit correctly, it has to be reduced to about 2.81m, but then it is inconsistent with the 3.0m room width.
The room sizes are internal measurements. For display purposes the walls centre line is used as its measuureing point so the walls thickness is not taken into account. The grid should not be used as a measure tool but as a rough placement guide. (Thats why you cant snap to the grid).
We are looking to take into account wall thickness in a future release, but this may add an unwanted complexity in drawing simple 2D plans, especially where the wall thickness varies.
Where you have a requirement to have an exact floor plan that already takes wall thickness into account and willing to exchange speed to create for more detailed plans, then you should take a look at our Visual Building product.