T8A, your space should be easy to work with. I believe that a version with O-72 for the outside loop and O-60 for the inside loop could be done in about 7' x 18.5'. A version with O-72 minimum (O-84 around the outside loop) would take about 8' x 20.5', with O-72 turnouts.
With a large table against the wall, you need to consider access for construction and maintenance. A sturdy table you can crawl on top of is one answer (doesn't work for me personally). Perhaps a plan with access hatches or removable scenery would work for you.
This size table might be beyond the limit of a movable table on casters, but I would give it a try, and if moving something that large and heavy turned out to be difficult, you are no worse off than if you had not made the attempt. Using lightweight construction techniques will help, based on 2" thick foam sheets for the surface.
