One idea I have seen is to make (or ask your dealer for) a cloth template of the piano you are interested in. Simple lay an old sheet or other cloth over the closed grand and trace the outline of the edges with chalk onto the cloth. You can use the same cloth for smaller grands as well. Then take the cloth home, and lay it on the floor, folding it in to the extremities of the chalk outline. It is not quite the same as having the actual piano in your home, but you can at least get an idea of how much floor space it will take up.