A single 12' gate would be plenty wide. Your focus will need to be on a way to close it 'quickly'.
A potential problem I foresee...with too large a pen/trap, is that hogs (at least the educated/pressured ones) will be reluctant to enter the trap very far away from what they know to be the escape route (read gate). The answer to that (as previously mentioned) is a 'walk through' pen (gates on each end). It is my opinion...that you can simply dispense with that option by making a trap that is 'optimally' sized.
If I understand you correctly, your plan is to make a large trap, with the bait area at the far end of the trap. So large...that it would allow you to approach the trap (un-noticed) and shut it manually? I think in reality...what you will experience are few hogs willing to go that far away from the gate (escape route). Instead...I would expect to see them milling around outside the panels near the bait area, possibly digging under after time.
Another possible issue involves your 'approach' to the trap. Even if you are extremely quiet, you will likely have hogs on the periphery to deal with and of course...if the wind in not in your favor you will be 'busted' hundreds of yards before you ever reach the trap.
You indicate that your plan (at least initially) is to shoot the trapped animals. That is fine, but in such a large area you would literally have to go in and stalk them. I don't know if you have experience with trapped hogs, but when it comes time to dispatch them (or load them), the less room they have...the better.
It would not be impossible to rig up a lightweight 12' gate to drop 'guillotine style', but it would still require you to come up with something that the hogs themselves trigger or that you trigger remotely.
Even so, do not expect to catch every pig. They become 'trap shy' very quickly, believe me.
One last thing, don't forget the all important 'Corn Arrow'.
Just get after 'em, we can use all the help we can get.