Well, Meg's just recently farrowed in her 9' x 7' ark. She had 5 piglets (little bruisers they are!) and kept them in for the first 7 days. All are still sleeping comfortably in the ark, the piglets now being 5 weeks old.
BUT... I didn't put up farrowing rails as I was concerned about restricting Meg's space for turning around. I got away with this with such a small litter and Meg turning out to be a brilliant mum, but if you want farrowing rails you may have to put them across the narrow width (ie, 7' rails) or, better, across a corner - if your sow has a big litter you may be thankful you did make them a safe area.
The piglets will be fine kept in for 7-10 days as HH says, but were you planning on letting mum out for a stretch and forage? If mum isn't free to come and go to pasture, remember to put in a sod of turf for the little ones and/or to give them iron injections. (Meg brought clumps of earth in to her babies.) Also, if mum is being kept in too, you'll need to have some spillproof way of providing quantities of water to her; she'll need lots of water to produce enough milk for her litter.
You will need to muck out their toilet corner regularly and make sure the bedding is dry and warm - I got caught out not realising the straw which looked dry and warm was in fact wet and cold.
Meg is an OSB gilt, 15 months old at farrowing - not a huge breed and she's not as big yet as she will grow. I'm not sure how much larger she'll need to be before I think the ark isn't big enough for her to farrow in. I also don't know how many growing piglets plus sow the ark would comfortably sleep - if she had 12 or 14 piglets the size these are now, it would certainly be very overcrowded in there by now. (But I guess a litter with two to three times as many piglets wouldn't grow so fast as these 5; these seem as big at 5 weeks as Meg was at 8 weeks.)