I use the resolution, but it says it has to be the same size for every room! How do I get two different sized rooms in the same game?
What exactly is the problem? Who says that every room has to be the same size? If you mean that note when you import a background, ignore it. It just says that the original room's size is different than the one you're importing.
The resolution (320x240, 640x480, 800x600, etc) is the same for all the rooms in the game (it's something you can pretty much set and forget), not the size of the rooms themselves. Resolution refers to how much will be visible on screen at any time - so basically, how much detail you can include in a single 'screen'. Rooms bigger than the resolution will be scrollable, and will follow the player if they move 'off screen' (like the town square in RotN games).
New rooms, before you add your own backgrounds, are created at the resolution size - as Mr Matti said you can just ignore the 'New background is different size' warning, especially if you haven't added any hotspots, walkable areas, etc. (And really, why would you have before you import the background :)) Just import the backgrounds ... Provided they're all AT LEAST the same size as the game resolution is set to it'll work.
(The exception to this is animating backgrounds - all frames IN THE SAME ROOM have to be the same size as each other, even if you're only animating a small part...)