On straight aways if you are running traditional stuff you could probably get away with three inches center rail to center rail, but if it were me I would go with four as a minimum and maybe four and one half to five if running scale stuff (not that that much scale stuff will run on 27). The problem will be when you get to the curves at the end, you will need a bit more space between them in turns and that means doing one of the two things shown below: top - starting the curve on the inside sooner than on the outside, both with the O-27 radii, and having more spacing on the straight connecting that end to the other side than on the longer straights, or bottom - both starting the curve earlier and using a wider radii on the inside.
