Table size for reverse loop

I would think for a 099” curve, the full table would be around a few inches wider and longer, to avoid losing an engine over the side. That would mean around 8’6” in width and length to accommodate switches. That would need to be repeated on the opposite end of the dog bone or u shape layout. Anything smaller should fit inside allowing for passing clearances.

You need a lot of space.  This one is 13 x 23 plus extra for the table for 72 inch diameter curves.  A figure 8  would not take up as much room (about 7x14) , but if you are going big what's the point? I was trying to figure out how big a room I needed for particular layout designs with 72 min curves.

You could save space with a Ross double crossover. 

Excluding the distance between between the track centers and the table edge the minimum width is the curve diameter.  The minimum length for the loop to return to parallel is 1.5 times the curve diameter.  If the loop is not going to close with itself but exit parallel to the entry then the required width increases by the desired separation.

For your O99 reverse loop with a 4.5"  track spacing between the entry and exit tracks is 99" + 4.5" or 103.5".  The length would be 1.5 * 99" or 148.5".

Here are some possible configurations of reverse loops.  These are done in O54 but they illustrate the idea.  The first example is the minimum as described above. By inserting 4.5" straight in the right side the entry and exit will be separate but parallel.

Jan