Dale says:

I’m not really sure what it is you’re asking so let’s take it step by step. Assuming that you are placing the track right at the edges. and assuming that you are using 18 inch radius curves, then the total track length will be about 190 to 200 inches. (Approximately 58 inches straight on each side plus about 12 inches straight at each end plus about 25.25 at each of the 4 corners.) This equates to about 4825 to 4840 millimeters total track length. At 3.5mm per foot (HO scale), that would be approximately 1370 to 1390 scale feet. Of course, if you place the track in an oval where the track is farther from the edges, the total track length will be less. if you go much larger, it will fall off the table.