Comfortably 30".

It depends how big your family members are and whether you are going to add a glass front wall. The glass takes up some of that 30” that Millworkman cites. Shower curtains take up less room. If you don’t have bath showrooms that have shower displays to try, mock up something with boxes. If you are going to have a glass front, then mock it up along a solid wall. Ask at a glass supplier how many inches into the shower the glass will sit, then make your walls that much closer than 30” and be sure to continue your mock up walls to your shoulders. It is arms that will be hitting walls, glass, and curtain.

As much over 30" as possible.

30” is too small for a 72” length. I wouldn’t want anything under 36”, and 48” would be more proportionate.

30 with a shower curtain, 36 with glass.

Thanks all!

The minimum required finished interior width is 30 inches, but 36 inches is recommended. If this is a curbed shower, then plan on the glass enclosure sitting in the middle of the curb. If you are planning on a pivot door in the shower, make sure that it won't conflict with an entry door or hit a toilet seat when opened.