From a maintenance perspective, I have found greens less than 40 foot wide (plus collar) tend to wear quicker than ones wider. There is just not enough room side to side to move the, say, middle pin.
From a play standpoint, with a 7% lateral dispersion pattern typical, a 100 yard wedge shot would require at least 21 foot width, but unless a par 3, we can't count on all players having that short a shot. And, in the great outdoors, it would likely look a little puny anyway.
I have designed narrow necks of greens (like the front of the T shaped Par 3 12th at Fortune Bay) as narrow as 35 feet wide, as long as the whole green isn't that narrow.