Most shower door units have a couple inches of adjustment allowed by the type of hinge they use, so decide how big a door you want( I'd go 27-30" for a shower this big) then look at the differences in quality among the price range for this size units. A Lowes or HD is a good place to do this, as they have multiple units on display racks, allowing you to operate the doors. You will readily see that the quality is directly related to the price. More $ will have thicker glass, sturdier extrusions (very obvious in the flimsiness of the door itself), and better looking and more durable finishes on the extrusions. If this is your first experience, I agree with the suggestion that you buy a door first, become familiar with the dimensions as explained in the installation manuel, then frame the RO, allowing for the thickness of the tile, the backer board and 1/8" for thinset on each side. A local glass shop can fabricate a door in any size you want, no matter how odd the dimension, but it will be more costly than a boxed unit.