Sean,
We have a 13 X 20 layout with an outer loop plus one spur of Std Gauge with O gauge on the inside - all on same level however. Allows us to run Std Gauge without interfering with the O gauge which is much more extensive. Smaller size layout should get same results. If you have any kind of spurs or sidings on the Std Gauge loop I would very much recommend Ross Std Gauge switches - I have one and it is smooth as glass.
Mike