Summary:
The size of the safe should be determined by the number of guns you possess, or are planning to purchase, and their sizes.
Protecting our office or homes from each and every burglary or theft is not possible. However safes provide additional safety to our valuable possessions, and always act as an added protection. Safes can be used for two purposes, firstly, storing precious items like jewelry, diamonds etc. and secondly, for storing, dangerous goods or items like guns, chemicals, bullets, etc.
People all over the world are killed everyday due to accidental gun shots. Such accidents can be prevented, by keeping guns locked in a gun safe, away from hands of kids, thieves, and other unwanted people. Fire proof or fire resistant safes also protect the guns, in case of a fire breaks out in the house.
Again, the size of the safe should be determined by the number of guns you possess, or are planning to purchase, and their sizes. Bigger the safe, more costly it will be, if you don't compromise on the quality of the metal. Generally people buy smaller safes if they have pistols only. The shelves in such safes are easily removable and hence you can easily arrange the guns in whatever manner you like.
The benefit of having smaller safes is that, they can easily be hidden under the bed or in the cupboard or somewhere where it is not noticeable. Safes for rifles and shotgun are bigger in size and are much more difficult to move, this is an added advantage, and provides a greater degree of security. Smaller safes should be drilled in the floor or the wall, if portability is not needed (in most cases it isn't).
You need to decide your budget before the purchase - Better the quality of the material, or higher the degree of durability, more expensive will the safe be. It is up to the owner to decide whether he needs the particular type of safe, and the best quality and degree of durability which he is getting for the price he is paying.