Obviously, if you intend to build a train layout, the larger the space, the better for future growth whiich will happen. Basement are a little like boats: All of them are two feet too short! Now, if it's a nice house and the basement isn't quite what your like to devote to your trains, there are a couple of things you can do: Build up; my own layout runs from 30" above the floor all the way up to 60" off the floor. My basement if one that is probably 20 feet too short! So I have four levels of track, all interconnected and trains run from one level to another.
Point is, if you have any space at all for a layout, you can make the best of it by doing a bit of planning and building.
Paul Fischer