With this labeled track diagram showing the 4.25" center-rail spacing.
The layout "closes" at 1/32" maximum tolerance everywhere (most closes perfectly) except the straight section between the bottom right two O60 turnouts (of the three O60 turnouts at the bottom), with that straight section of two 10" and one 1-3/4" being short by about 1/32". Anyway, very close to perfect, and the layout will (should) assemble without gaps between sections, or any compression or twisting of section joints.
As previously mentioned, the track runs right to the edges of the 4x8 sheet, so the outside frame of 1x3 or 1x4 adds 3/4" (1.5" total) to the length and width, giving 3/4" of space for clearance for any cars or locomotive overhang.
The small sections of dark-blue need to have the plastic roadbed trimmed (easy to do), to fit with with the O60 turnouts without the 1-3/8" "fitter" sections at those locations.