I honestly would try and run a around wall style layout, atleast 2 feet width, maybe even adding a peninsula to put a big industry on or something of that nature. this will give you the longest possible mainline run and have a ability to switch at industries and to make a yard...
There is quite a bit of layouts that others have done or made designs of, if you use the search button you will be able to find a few. this forum has a big database for finding info from the search function
Track spacing depends on a few factors of what scale you plan to run, you havent mentioned. It also depends on what rolling stock you plan to run, freight, passenger cars, what would you like to run, its up to you? When you figure that out you can plan track spacing