There is no maximum width for a roman shade.

If you plan on making a roman shade wider than 50" you will have to piece your fabric together. This is where it can become a bit cumbersome. If you need to piece your fabric, be sure to piece with 3 panels... not just 2. The last thing you want is your seam smack in the middle of your shade. Plan your seams where you will sew your rings. And have the largest panel in the middle with 2 smaller panels on the sides.

You can get instruction on how to piece your fabric in How To Make Roman Shades.

As far as functionality goes, I advise to install cord idlers instead of screw eyes. This will reduce the friction on the lift cord.

And if you plan on raising and lowering your roman shade frequently, consider installing a soft shade clutch system. You can learn more about this at www.roman-shade-supplies.com.