Years ago there was a booklet with title something like "60 plans for small model railways", by Peco. I think that it is still available. There are plenty of other books with plans.
Your size is a little odd in being longer but narrower than most published plans which are for 6ft by 4ft, 7ft by 5ft, 8ft by 6ft or 10ft by 6ft, however, this shouldn't matter too much. It is far better to expland a plan for something like 6ft by 4ft than to try to compress a plan for a larger site than yours.
You have just about enough width to run a complete oval is that is what you want. On the other hand you don't have enough width for a central operating well. This means that you will have to operate from the outside. Seeing the whole oval in front of you can make it seem toy-like. You might want to put part behind some scenery.
Good luck with the project.
