A lot would depend on how much air you have coming into the booth. Think of it as an exhaust system of a car. A car's engine needs back pressure to run properly. When a cross over pipe or H pipe is installed it evens out the back pressure. If you run to large an exhaust pipe or pipes you don't have any back pressure even if you have an H pipe installed and the engine won't run to maximum efficiency.
The amount of air coming into the booth will determine how large your exhaust pit should be in order to have positive pressure in the booth. You want positive pressure inside the booth so that if you open the booth door, air will escape from the booth. A booth is set up this way so that in the event that there are any openings, ie. seals between the booth entrance doors, the man door going into the booth etc., the air is being pushed out of the booth and air is not being sucked into the booth. If air is sucked into the booth, or in other words the booth has negative pressure, the air coming in could have dust and air born dirt particles which in turn could land in your fresh paint job. The larger the exhaust pit the more difficult it is to get positive pressure, the other variable that controls pressure is the amount of air that is being moved by your exhaust fan. The exhaust fan needs to move slightly less air out than is coming in. That's why dampers are installed in the exhaust system in order to balance the booth to a positive pressure mode. There needs to be a balance between the amount of air coming into the booth, the size of the exhaust pit (system) and the exhaust fan.
So in answer to your question, if your running a 6 foot wide pit first of all, it would probably be to large for maximizing positive pressure and the sheet metal divider wouldn't allow enough space for evening out the exhaust flow. In the H pit design, again depending on the amount of air coming into the booth, should be about 2 to 3 feet. In what you described, it would be less than 1/16 of an inch and you wouldn't get the benefit of an H pit design balancing the exhaust flow. One thing that hasn't been discussed is how deep is your pit...this isn't as important as the width, but does have relevance.
If you can give the air flow numbers as to amount of air in versus the amount of air out, I could give you pit dimensions that would suit your specific needs.
I know it can be confusing, but I hope this answers your question. If this is still confusing, let me know and I'll try and explain it in a way for you to better understand.