I have to be honest and say that you will not able to do much with that space. Even with a cockpit design where you have to duck or lift a bridge to get to the central operating pit, you will face some challenges. For one, your pit, if it is to be large enough to move and swing your arms without contacting the bench with your elbow, will take up a lot of room. In that configuration, you are left with little more than a roundy-round, a loop with perhaps a siding or two and a couple industrial tracks. You will have little room, if any, for servicing engines. Also, grade elevation changes will be a problem in that confined space. You will very likely have to make it a flat layout, or live with grades in the 4-5" range and get used to either short trains or double-headed ones.
Too bad you couldn't make it a single slab around which you could operate from all sides. Even at 5' wide you could always walk around to the opposite face and reach something. You'd have more area for a small village, even more room for gentler grades if you used a figure eight track plan of sorts. Maybe a servicing area for a bit of variety, and even some industries.