How much space is needed to hang a heavy bag?
Ideally, you really do not want to feel confined when you are whaling on a punching bag. You do not necessarily need acres of room but you will not enough space to feel comfortable. At the absolute minimum, ensure that the bag has enough room to freely sway back and forth (both on the sides and at the bottom). As for the specifics, you can determine that based on your preferences and what you can achieve practically.