They do well in a regular-sized x-pen, and even a double x-pen (two attached for double space).  When I open up the x-pen, no matter how much space I give them, she starts to chase.  So, even If I close them off in one section of the living room, if she isn’t surrounded on four sides by an x-pen, she chases.  My living room isn’t very big, so I’ve tried 1/2 and 1/3 of the room.  I’ve also tried different sections of the room.

It slows down once he is far enough away from her again.  So they’ll flop on opposite ends of the space.  But if he ventures too close, she chases him back.  Almost like she wants to divide the space into her territory and his.

She doesn’t do any of these behaviors in the x-pen. She shares litterboxes, food and water dishes, and hidey boxes with him in the x-pen. They snuggle often, and groom each other. His behavior in the x-pen shows that he is comfortable and confident in his place in the hierarchy with her in the x-pen. The x-pen they live in when I am at work is in her former territory in the bunny room, which theoretically should be the place she would be most territorial. However, when I moved them from an x-pen in the living room to the bunny room after bonding them in the living room, she didn’t even bat an eyelash. Nothing changed in her behavior towards him. It’s only free range when she has territory and hierarchy issues.