I agree with the go as large a curve diameter as possible. When I started building my layout, I thought I would never need anything larger than 42 inch as that was the minimum for the larger passenger cars. Then I found myself buying a Burlington Zephyr that needed 72 inch curves. Fortunately I was able to fiddle around and managed to get in the larger loop. I now can run 3 trains on separate loops, or I can run almost everything through all 3 loops. All squeezed into a 9x13 space.
Of course then I decided it would be much easier to be able to leave passenger trains on the rails requiring some sort of staging yard or siding. With a 5 car passenger train being plus or minus 10 feet long, this has become a stopping point for me because I do not have the room to add this length of track. This is where an under table yard could have been a real solution to my space issues - if I would have thought about it at the very start. As it is, I can keep a couple of short consists on the sidings and just add a 2 or 3 coaches when I want to run a particular road name.
Lastly make sure you add the extra yards or sidings to hold your rolling stock and engines. Looks to me you have enough room to fit in a lower level yard (at the front left) if you put in a turnout around the top right and run the decreasing slope yard track to the left and then under the mainline. You probably would need to gain an inch or so of height on the main at its top left location to gain enough clearance to keep the grade around 3% or less. After that you could work in a decent sized yard.
As Matt said - plan, plan, plan ... then I will add figure on changing it again because you will decide there are better ways to run your layout than you originally thought.
Have fun - Jeff
