Regardless, you will most likely want a room at least 10' deep to be safe! Once again depending on your body and swing. Most screens and nets are around 3' deep and the average driver swing will span somewhere in the neighborhood of 6'.
The depth of your room needs to provide enough room for your net or screen plus the room it takes to swing a full swing with your driver. It is important to also take a follow through as often times a follow-through swing reaches a taller height than the backswing. But it's best to take out your driver and take some slow motion swings in your potential golf simulator space to be sure you are in the clear. The highest point in your swing will depend on your swing and how tall you are. The height of your room needs to be taller than your net or screen and taller than the highest point of your swing with a driver.
The width of your room should be at least the width of your golf net or screen plus a foot or two to be safe.
