Nope, a safe room is not a small cubby hole to deposit a safe in. A safe room is a small room for you and your family to run into when the bad guys are breaking into your home, where you can bolt a strong steel door closed and keep them from getting to you and doing bodily harm to your family. Its size will be determined by the number of people who will be using it and how much space you are willing to delegate to the room. It should be large enough to store water and food items as well as some emergency medical supplies. The room should be constructed using fireproof materials such as concrete block so your uninvited house guests can't burn you out or ax their way through the walls. If you use 8" wide concrete blocks and fill them with grout you will have a bullet proof room. The best way to design and construct a room is when you are building a new house. This way you can build the safe room and construct the rest of the house around it . The room should be centrally located in the house if possible so that the other members of your family can have a chance at getting in the safe room without having to run to the opposite side of the house. The entry door should be faced away from the front door to the house. If you already have a house and want to retrofit a safe room, this is more difficult and will take more time and labor. You might have to sacrifice a small bedroom or remove a wall in a hallway to gain access and get the building materials into the room. This will also require a hole to be cut into the existing wood floor and a footing to be poured in the crawlspace or in the basement, whichever you have. This undertaking would be a monumental job and your house would be a construction zone for a couple of months. A third option would be to build the room in an attached garage where you could have better access to the project and keep the construction zone and associated dust and debris confined to the garage area.
In the midwest and plains states, many rural homes have an underground tornado shelter for family members to get in during a tornado. Most of these small rooms are built out of poured concrete or concrete block with a poured concrete roof, all tied together by rebar so the roof can't be pulled off from the forces of the tornado. This is basically the same design an above ground safe room should have. The ideal safe room could also be used as a fallout shelter if the need should ever arise. You can stack sandbags on top of the concrete ceiling of the room to get the required 2' of earth over the roof to protect you from gamma ray radiation.
One way to get an idea of how to design your safe room is to look up the old Civil Defense fallout shelter designs. Some of these are downright comical and others are very well built with cut away drawings and material lists provided, to give you an idea of how much the materials would cost in todays money.
If you choose to use concrete blocks remember that they are 8" wide and 16" long so if your room is 8' long it will take a 6 block run per wall. You must remember to use dimensions that are divisible by 16" so you won't have to cut any block to make them fit.