It really depends on the number of tracks that you intend to lay on each section and the track spacing. My layout has both 2' and 3' wide modules. Atlas O is designed for 4.5" spacing so 4 tracks would consume 18" leaving only 6" for scenery and centering.
You don't have to have a constant width along a side. Bump outs of any width are possible while still keeping things within reach. check out Norm Charbonneau's postings.
Also, you need to take in account the space needed for large diameter curves at the corners. I had to add triangular models to accommodate my O81 and larger curves. You have a 16' width which is 2' wider than mine, and I am using O90 and O99 curves for my mainline.
With your width you can use the middle while still maintaining generous aisles. I have a 4' wide yard with 2' and 3' aisles.
Jan
