RMk:
insideL = wall.lenght() - 0.5*leftwall.thichness() - 0.5*rightwall.thichness()

On straight walls,
on circled walls : http://en.wikipedia.org/wiki/Arc_(geometry)

Not that simple, this length depends on the angle with the next wall, the thickness of these walls and whether the room you draw has a concave or convex shape!
By the way, note that while drawing it's impossible to guess this information, since the program can't anticipate if the user is going to create an other wall after the one he's currently drawing.