I recommend nothing smaller than 4 feet by 4 feet of floor space — plus lots of run time out of the pen like you allow already. (smallanimalchanel.com)
Of course, when you're home, the puppies come out of the pen to be with you. (sardihavanese.com)
I would like to see some free agent starters signed, drop their worst pitch and see what might happen out of the pen. (sbnation.com)