not big but the Interior sets like the living rooms which ive made are probley the same size as gromits bedroom set etc. i usually design the sets how i want it to look like build sets from wood, then add the details like lights and furniture.
i think you should get a wallapaper pasting table (from B&Q) they come in 2 sizes a short one and a longer one short ones are perfect for the "interior sets" and the longer one is great if you want a "outdoors set"
also when building a set measure the height of your characters, and make the walls of the set higher so that your camera and characters are not higher than the set.
if you want to build a set which you can take apart easily and move around, i would use foamboard or (Foamcoreboard) as this is light and easy to cut it comes in mnay different sizes ,Staples ofiice place has it in packs of a3 and a4 these are perfect for walls, but make sure you use a wood floor and stick the walls around the wood floor. anymore questions just ask. 