Most starting enclosures are several acres........that said, an old man took a san Jaun and put it in an enclosure that was about 50' X 50'. He put the pup on a 50' lunge line an got the pup chasing the rabbit. After a couple of minutes he opened the enclosure and let the rabbit out then unsnapped the lead on the pup........pup started chasing the rabbit.
Most of the guys take their hounds to some one with a several acre enclosue with wild cotton tail and want the pup back as soon as he barks on a line. To much enclosure time, unless it's a large enclosure, creates a host of different problems. If there are a lot of rabbits then the pup might learn to let go of the line he's running in order to jump a fresh rabbit thus avoiding the hard work of regaining a lost line. The real problem with small enclosures is to much scent. Also, enclosed rabbits, even wild cotton tails in an enclosure, run different than in the wild. It's not hard to tell a hound that runs frequently in an enclosure, so most guys want the pup "green" started.
I think the best set up I've seen is an acre enclosure with plenty of rabbits to just get the hound barking a line and then moved into a 50 acre enclosure with fewer rabbits for a few days before going out in the field.
Some enclosures have deer in order to persuade the pup not to run a deer line. Myself personally, I'd rather keep my hounds from smelling a deer until they are well started on rabbits. A beagle is a multi purpose trailing hound and was bread to run anything. With the deer as thick as flees, most guys are trying to breed out running "off" game...but are meeting with limited sucess. Several more generations will be required in changing such in-bread traits and it's very difficult to find hounds that won't run deer. I learned a long time ago to say they haven't run deer instead of they won't run deer........a hound has a mind of his own and if I can get a pup up to age 3 without running a deer there is a good chance that they won't run deer. Deer run several miles. The daily limit on doe deer in middle TN is 3 per day......there are a lot of deer and hounds running deer, although defined in the AKC rule book as not a fault, is one of the biggest faults a hound can have in the eyes of most hunters. There has been an effort to change the rule but it hasn't happened yet.....
I also like to run the pup solo until he can consistantly circle a rabbit by himself before putting pack pressue on him.........to much pressure to early and you can "blow" the pup up, or in other words, he begins to run without regard to the line, becoming over competative to a fault and has to have the front.......after he's doing it all by himself I like to put another hound of equal ability with him and slowly work up to a pack after he's deminstrated he can handle the pressure.