What size basement you should be looking for has many factors. First of all, what size equipment and minimum curve radiuses do they require? How much time do you have (and money) to construct a layout, be it large or small? How much equipment do you have (never enough)? What type of house are you looking for, ranch, colonial, cape cod or split level? The type of homes you look at sometimes dictates the size basement that you end up with.
Most important, is the basement dry AND deep enough? A shallow basement does not leave much room for lighting, finished ceiling, etc. A wet basement is not good either. Dampness is a very serious enemy to our hobby. You don't want your prized possesions to get rusty or the boxes to get damp and moldy. It is quite expensive to repair a wet basemnt.
Is the area in the basement whrere you are planning to build your layout finished? Is there a ceiling with adequate lighting? Are the walls finished? What about the floor, is it carpeted or atleast tiled? AND, what about your electrical requirements? Are their enough outlets and hopefully a dedicated circuit for your train room/layout? The cost of all of these things can add up quickly and become quite expensive.
My wife and I bought our current home in 1998. Like alot of posters before me posted, what was above the basement was utmost on our priority list (darn!). We bought a four bedroom/3 bath spilt level home because it was a very nice sized home for us and our four children. What I got in exchange was a rather smallish FINISHED basement room which could also serve as a 5th bedroom. It has a walk-in closet for my train storage and is carpeted, finished ceiling and walls, and a light switch controls one wall electrical outlet. That is where I have my power strip and transformer plugged in.
Happy house hunting! I hope that this helps. Keep those future rails polished!
Cobrabob.