There are plenty of auto's out there with many of the makers offering a youth/womans model with dimenstions for the smaller shooters.
The 20ga. with 7/8 oz is plenty of gun for any upland game and doublely so if hunting over a dog. The Bennelli auto is very light but I'm not sure if it come in a youth model. Beretta has a short stock available and thier guns cast and drop can be tweeked some with the shims they come with.
If you shoot over a dog and keep shoots under 40 yards quality target loads in 7 1/2's would do for most birds. For pheasants you might want to go to a #6 for a second shot but you don't have to get the hyper velocity heavy stuff. The normal standard field load is plenty.