As with any important item of clothing, the fit of a shotgun is equally as crucial when getting your first hunting firearm.
In the mainstream world of shotgun hunting, you have upland, small game and furbearers, and waterfowl categories. Within each of those categories, you break it down by individual species and the particulars for hunting those animals.
For upland hunting, you have dove and quail on the smaller side of the scale, chukar and ptarmigan in the middle, and prairie chicken, grouse, and pheasant on the larger side. With upland hunting, you can successfully hunt all species with a 20 gauge gun, but you will need to adjust your shot size depending on what you're going for. (See my separate post about ammo and how to understand all the crazy numbers on those boxes of shells!) A 12 gauge can also be successfully used for upland hunting and can ensure that you'll be able to reach out further with some of those longer shots.
The species within the small game and furbearer categories are many. From prairie dogs and squirrels to rabbits, porcupines, and beavers - habitats and sizes vary greatly. For the most part, a 20 gauge will work (again, need to plan your shot size and ammo depending on the specifics of the animal, as well as the distance of the shot). A 12 gauge might be too much for the smaller animals, but can ensure clean takes for the larger ones. I consider turkey hunting its own category, and depending on the species of turkey and the distance of your shot, either gun would work, but I'd be more comfortable with a 12 gauge over the 20 gauge.
Waterfowl is where some might say that the best gun is the 12 gauge. I've shot a 12 and a 20 for waterfowl, and for me, it comes down to the species of bird I'm after. With ducks, my 20 gauge has been the perfect tool. With the right shot size (I prefer 3-inch 4-shot ammo), you can successfully take ducks of various sizes. With geese - it's a totally different story. Especially with Greater Canadian geese, the birds are BIG - and heavy! A 12 gauge, and even a 10 gauge, are ideal for those larger honkers (I prefer a 12 gauge with 3-inch or 3 1/2-inch and either 2-shot or BB ammo).
In many cases, it's tough to afford multiple shotguns. If you're just getting into hunting, it can be overwhelming the type of gear required, and the price-tags that accompany all of it. It's important to learn the basics and get comfortable before purchasing your first shotgun. That's why we provide 12 and 20 gauge guns for participants to use at our upland clinics. They can test them both out and see what feels more natural to them.
If I had to choose a size for a lady to start with, my gut choice is a 20 gauge. If you want to do multiple types of hunting, a 20 gauge provides great versatility, and it isn't as heavy as those 12 gauge guns! Having a lightweight gun is ideal for pushing many miles in the field and for multiple accurate shots in the blind. Tired arms are not good for making accurate shots!
Bottomline though, try them out and see what feels right. Learn about the versatility your gun can provide and always, ALWAYS make sure you're shooting with the right ammo!
Happy hunting ladies!
My current go-to gun is a 20 Gauge Semi-Auto Weatherby, 28-inch barrel length, synthetic stock. It can take up to 3-inch shells. Extremely lightweight and user-friendly.