Hi Stuart
This is a track plan of our smaller roll-away Scalextric Sport layout - I think it's around 6ft long by 2ft 8in deep:
If you have 6ft depth as well you could extend the track by inserting straights into the middle of corners "Samson" and "Duck" and then run straights leading to and from "The Rumbles" the full length of the board again.
Hope this helps give you some ideas