Morgan,
Our Arcs are a pent roof design. In an older one the lowest height was 3'6", this was raised to 4'3" when our biggest Tamworth boar pushed the roof off from inside. The entrance to the Arc is 30" and there's no bother with him coming in/out, just make sure that the sides of the entrance are strong enough to withstand a good rub - we use 3x3 timber uprights as the "door frame" and the most they have rubbed off them is 3/4".
Cheers
Gavin