It all depends what you're going for.
For tours on a cd-rom (my main type I do) I use
8000 x 4000 and trim it to about 2mb
Web tours I'd go for
4000 x 2000 and about 1.3mb per scene - but this really depends on how many scenes you want, the more you have the longer its going to take to get working online.
How many scenes? I do virtual tours of historical places with text and audio so each of my scenes is more than just a 'look around'. I tend to think that the very least a person will spend on a scene is 2 minutes. If you have 15 scenes then that is about half an hour, at least. Some of my tours are 100 scenes or more so I split these up into around 6 smaller tours of around 15 - 20 scenes.
Does this help at all?
I would imagine that a virtual tour for a real estate / realtor would be considerably different however. My tours are quite specialised so please bear this in mind.
Yell if I can help any more.