I like to have a large pen for two or three ewes with their new lambs. I use it as a staging area between the mothering-up in a pen on their own and before going into the big outdoors. Any ewes and especially first-timers and adoptions where I am not completely certain how well joined up they are but don't want to keep them in a single pen any longer. It does need to be quite large - 15' x 15' or more - so that those ewes who can be aggressive to lambs not their own do not feel pressured and harm any of the other lambs. I also train lambs who need it to come to me for a top-up in this area before they go outside - mum can interfere with catching them to give them a bottle once they are outside, but if the lamb knows about the bottle it'll run to me irrespective of mum's opinion.
Personally I don't pen the ewes in an individual pen until they have lambed unless there is a reason. It restricts their movement while lambing, increases the risk of squishing one lamb while trying to produce the second, and you get all the birth mess in the pen which therefore needs immediate mucking out.
In an ideal world I like the lambing pen itself to be not much deeper (front to back) than the ewe is from nose to tail and just a couple of feet wider than deep. I like them to be twice as wide as they are deep for triplets. I like there to be a safe place for the lambs to get away from mum, if not in every pen then in a few for those ewes who get aggressive, for any adoptions, and for triplets. A shelf the lamb can climb onto works fine.
I like the lambing pens to be very easy to clean out.
I do agree about making the pens temporary so that the barn can be used for other things the rest of the year. A system I had on a previous farm and am planning to put in here is to have batons along the back wall with brackets to which the pen sides will attach (by pole through the holes in the brackets and matching holes in the pen sides.) It's all very easy to put up and take down and very sturdy when built. You can either buy pen sides with the holes in (some metal hurdles will have holes for poles to connect to each other) or you can attach brackets to your own wooden pen sides.
I also like to use something to hold the hay so that they don't waste it and get it all twisted around their feet. A flap of weldmesh is a cheaper option than the clip-on hay racks.
Depending on the number of sheep you are looking at and their breed, if you are planning to feed cake in the shed then you may want to consider a setup whereby you can put out the feed in the troughs / feeders before you let the ewes at it. Unless you run very fast....
