Is there a required shower door size, i.e., minimum 24 inches wide?
No there isn’t Peter, that I know of, or read in any code book…“nada”.
Depends on the “width” of the client. 
True…I do know some folks who would not fit through there even greased—
I thought I heard a greased pig could slip in any opening. :mrgreen:;):)
Thanks for the visual Dale!
helps if they open outward, too
less than 20" on this one :shock:
Like a sauna, a shower door should open outward for safety reasons.
I guess you got in the shower to test this one…
I am a bean pole, so entry/exit was not a problem. Opens outward.
It’s new construction, so I was concerned, and recommended review of the plans, etc, etc, etc.
Richard M was close–there is no door minimum, but the maximum user size is directly proportional to the entry opening.
Well, 14 years later, I found a small shower door opening of 18 inches, and, yes there is a minimum size of 22 inches.
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Bet that kept you awake at night! 
You would think, but nope, not at all.
Philippians 4:6
I concur, Peter. 22" inch wide for a shower stall.