There's no clear cut answer for this - it depends on how much of your rooms you want to be visible at the same time and what scale factor you're aiming for (not to mention adaptive scaling based on different resolutions and aspect ratios).
The best advice I can give you is to grab a tile set, build some stuff with it and experiment around with the view size until it feels right. Look at other games which use similar tile sets and see what resolution they use. Grab a couple of screenshots of different setups and post them here if you're having a hard time deciding which one looks best.