Generally, you need things to be a bit wider by a few inches than any given track size.
It would be very helpful if you could provide a simple drawing (pencil or otherwise) and/or a photograph of the overall layout. You have identified a few variables and it's difficult (at least for me) to be confident in the interpretation without some kind of visual aid.
Others, more competent than I will chime in... shortly.
Best Wishes and Happy Railroading!
