8x10 would be ample for a boar and two sows I'd think, I'm sure we had 4 LB's all in one that size. We had two sows farrow next to each other in the shed (they'd been running together all through their pregnancies) and we put them back out together, with their piglets (they were around 10 days old) into a field with two arcs and yup, they ALL piled into one
Having another arc, 8x8 or 8x10, along with your existing one and the space in the shed should give you enough flexibility. But you'll have to factor in housing the piglets separately for weaning (at least 2 weeks) before they go back alongside the sows, assuming that they are going back, or do you have another area in mind for them?
We tended to bring the piglets back in to the shed at weaning - that way they didn't try to get through/under fences and back to mum. Mum couldn't see them and was quite happy with a bit of space. It's also an awful lot easier to catch weaners in a pen than it is in a field when buyers come to collect
In-pig second parity sows would be the easiest way to start, they'll have done it before so have an idea of what to do when the tine comes. I'm not sure where the closest Tamworth boar would be though - I think there used to be one at the museum of rural life in East Kilbride.......ask the breed club /BPA and do your homework to check he'd be suitable in terms of bloodline and size for future matings - we spent a lot of time travelling for boars after a few failed AI attempts (and we had a teaser boar on site) - you ideally want to make it as easy as possible to breed them, without needing to keep your own boar (lots of extra expense unless you offer him for stud - and that's another post in itself
).
HTH