|This article is about an older version of DF.|
Sparring requires enough clear floor space inside the barracks to dodge and maneuver in; 3x3 is a safe lower limit.
When a bedroom is set as a barracks, dwarves will not claim the room when they sleep. If no beds are available, they will sleep on the floor of a barracks. If that isn't available, they will fall asleep where they stand, potentially leaving dwarves in dangerous locations. Military dwarves may be ordered to sleep in a barracks, even if they have a bedroom.
If your barracks is full of damaged, bloodstained clothes, this is because sparring dwarves have ripped them off of each other while practising their wrestling. Presently, there seems to be no way to get rid of the clothing, aside from killing the owner (and his/her spouse, if any). More importantly, wrestlers will occasionally spill the content of backpacks. The latter can result in rotting food that can not be moved because it is owned by a dwarf (who doesn't bother to collect it). It might therefore be advisable to make barracks a room that can be locked up to avoid dwarves getting in contact with miasma.