Go 12 feet X 3 feet.
You will be able to get in the middle of the bed easy 4 feet is too wide I made mine 4 and wish I hadn't.
Here is a grid I tossed together so you can see how to lay out the plants.
Stagger them a little on each side to get more room.
You should be able to get I think 10 plants in it.
Worth
Bed2.jpg