Gun fit is a critical element, and the average off the shelf shotgun is just not going to fit smaller shooters. My wife is fairly petite, but still handles a properly fitted (12.5" LOP stock, field length forearm) 12 ga. 870 with zero difficulties.
Still, a 20 gauge version of the same gun is what my niece chose when she came down for a session of Defensive Shotgun 101 a while back. Off a table loaded with various shotguns which she had a chance to handle and shoot, she chose a 20 ga. 870 Express Youth model (13" LOP, 21" VR barrel). Interestingly enough, there was a 12 ga. 870 Express Turkey gun on the table as well - 21" VR barrel, with a wood stock shortened to 12.5" LOP. My niece liked the 20 ga. better because she said it was easier for her to maneuver. So we gave it to her.
Weight in front of the support hand feels much heavier to the shooter. Thus no magazine extensions or other bolt-on stuff out there. A premium recoil pad is a big help. A field length forearm means the support hand doesn't have to be hyper-extended, making the gun hard to support and operate. And if the Youth version of the 870 Express is still too long, there's a Junior version with an adjustable LOP stock and an 18.5" VR barrel
Forget the pistol grip only stock...