A single sow needs at least 2x3. You add one grid length for each additional piggie. So .... at least a 2x8 (or a 4x4, 3x5 would probably also work). However, if you can do larger, I would.
You could do a 2x8, a 3x6, or a 4x4. You could also make an L shaped cage if that'd fit in your room better. I have a 3'x10' cage for my 4 girls (custom, so not standard grid sizes) and it's not too hard to clean, the bigger the better and the happier they'll be
If they are very active a 3x5 would be best, lots of play room while others are being lazy. Now my 6 are pretty much lazy piggies, I shrunk their 3x5 down for my back(medical) to a 2x7, a L shaped 2x3 put together with a 2x4. It's 5 sows and a neutered boar, only two younger sows that run around now. The only really active times are food times.