Good advice so far. The problems with a wide AND long slab lies in the reach. You'll have to use one hand for support, or lean your belly hard against the edge of the slab to get a long reach, and that's often where troubles come. The slab must be stable and well anchored if you're going to get a knee up on it, or get your belly hard up against its edge. And that's only possible if you only have access to one side and the other two sides are up tight into a corner of the room...where you can anchor it to studs and the like.
The other main problem comes from the curve radius. If your trains can handle small radii, say less than 20", then you can have a decent track, even a loop, in 5X7.
The last problem will come in the 'legs' of the track plan. A more interesting and complicated track plan will generally have more 'legs' than a simple loop and a siding or a spur or two. Those get old quickly. Track plans with legs hold interest longer and won't be so boring.