Check out the thread How do I start a Hostel? - New Hostel FAQ Under the heading "How should I arrange my hostel" there are several links that might be helpful.
A short answer is that for a comfortable spacious room, consider about 10m2 for the first bunkbed footprint, and 5m2 for each additional bunk bed footprint. So an 8-bed dorm would be about 25m2 and a 6-bed would be about 20m2. Obviously this would depend on several factors: the arrangement of doors and windows, lockers or other furniture in the room, the size of your bunk beds, how comfortable you want the room to be, etc. Plenty of places put the same number of beds in a smaller space too, but this is a decent rule of thumb. Depending on your area there may be specific requirements for air space for each guest and how the beds can be arranged as well.