How big is big enough? An interesting question to which each person will have their own answer. I am working on a 32'x32' N scale layout. I enjoy operations and operating my layout with friends. Although I have most of the layout built, I do wish my mainline runs between towns were longer, but I wouldn't want to give up any of the towns.
Try to meet some other model railroaders who have layouts and visit them. This can be done easily at NMRA regional and national conventions. Some of the NMRA divisions have periodic open houses or layout tours for there members.
I would also recommend getting some track planning software like XTRKCAD. XTRKCAD is a freeware program that will allow you to draw trackplans in each of the spaces you have available and better evaluate what size you want. Be sure to at least sketch in some scenery to make sure that your vision will fit. Some people even go so far as to build a model of the layout the want so they can see what end product may look like.
Keep asking questions here. Also try googling model railroad layout planning and see what sites come up.
Waltzing Creek & Western Railroad