Well a room's dimensions are 800x608 usually, this allows the 32x32 grid to be perfect, most people use the view 800x600 though. Anyways, you want your view to be 800x600, and under the settings tab, you just need to expand your width and height
Add 800 to width if you'd like to add a room to the right
Add 608 to height if you'd like to add a room below
Use the background to refer where each room is, unless it's seamless of course, you'll have to use a reference mark outside of the room
I'm just assuming this is what you mean btw, it probably isn't