With gun safe capacity, bigger is better. Most folks make the mistake of purchasing a safe as large as they need, or maybe a bit larger. You WILL fill it up. More guns will come. Insert a small document safe for important papers and electronic files, add a few handguns, maybe in their boxes or in an accessory rack or stacked in unzipped soft cases. Some jewelry or photographic equipment, coins, and the next thing you know, it's full.
Maybe you're concerned about the weight of a large safe. Don't be. The square inches of a safe bottom spread the weight evenly, the same as a refrigerator or waterbed. Your floors and stairs will hold up fine.
Maybe you are trying to save a few bucks. I understand. Most gun owners have one safe, and it is filled. Their alternative is to buy another one, or remove items to fit more guns. Buy a substantially larger safe than you think you'll need in the first place. You will fill it. Also, just because a safe's interior may be designed for 8 or 12 or 24 long guns, beware, you'll never fit that many in reality without bashing them together anytime you need to remove one. That figure of 8, 12, or 24 guns is based on small, slender rifles like a Winchester 94 lever gun or a field shotgun. In reality, we all have a lot of guns that have protrusions everywhere, like scopes, bolt handles, bipods, slings, shell carriers, etcetera. Besides, many guns are just very large, like tactical rifles with giant stocks, and AR-based guns with their pistol grips and magazines that just love to scratch their closest neighbors. An honest scratch on a gun is when I fall while hunting. Not simply by trying to remove them from a vault. Guns in close proximity to each other get gouged and dinged, even when you're extremely careful. Give yourself some comfort room.
Safes are generally made so they can fit through standard doorways, about 28" with the handle attached. Measure your doorways first.
Buy a safe one time, and make sure it has the long gun capacity and volume for everything you may want to store in it, now and forever. If you want to also store ammo in your safe, plan for it to take up LOTS of room.
Browning has a DPX system which allows you to store smaller long guns and hand guns on the door for easy access.
The height of a safe is up to you, but taller safes generally have more room on the top shelf. This is extremely useable space.