My rails are 2x3 mounted with a 29 inch outside of rail to outside of rail demension. This gives me enough room for a 17 inch section width.
Like I said, that is less that optimum, and I would go 24 inch os to os next time to be able to run any tire I want. Fronts are an issue. I run a 25 inch tall tire, and that is all that will fit. I can't remember what fit well when the front was stock, but it wasn't very big! I can't remember if I had to trim the front lips of the fenders for stock suspension and the 25 inch tires or If I did that after the chassis upgrade.