If you use Ross O-54 track for an inner loop with one of their fitter pieces between the switches connecting the inner and outer loops and use Ross O-64 track for the outside loop, you can fit both on a 6 foot wide tabletop as I did on my last layout.
The "fitter pieces" helped to make the curve clearance work perfectly for my locomotives (e.g., K-Line switcher, MTH FP-45 & ES44AC).
Here's a view at the other end of the layout where you can see the "fitter" piece between the two switches. The fitter piece is the connecting track piece with no switch guardrails. This was the key to getting the right curve clearance to fit O-54 and O-64 on a 6 X 15 foot layout.
I was considering an O-72 outer loop for my next layout, but since I'm not interested in the behemoth locomotives like Big Boys, I may stick with the O-64 outer loop. My arm's length is one issue as I found reaching to the halfway point on this 6 foot layout was a challenge, even on a step stool or step ladder.
I hope this info is helpful.
P.S. I used Ross Roadbed with homasote and 3/4" plywood. If you use Gargraves # 4 3/4" screws, they won't hardly show on the ties and will not puncture through both the roadbed and homasote to the plywood. I never had the "rolling thunder" sound problem from screws going from the track through to the plywood.
