I agree that Pine Snakes would benefit from as large of a box as you can find. I have kept a lot of bullsnakes which is where my opinion comes from. I have also kept pine snakes but am not as experienced with them.
One thing I'll warn you about up front is to get the box you think you want to use and keep a snake in it for a while. Most of the larger boxes on the market have very thick rims around the top for structural support. Then they also have sloping sides so the boxes can be nested inside of each other for shipping.
What both of these lead to is much less floor area than you would expect based on the listed dimensions of the box. For example, I use the Iris CB-80 tub that is available through Reptiletubs.com (great company, by the way). The listed dimensions of the box are:
31.5" x 17.5"
The actual floor area is:
27.5" x 13.5"
The the box is 4" shorter and 4" narrower than the listed size. This can be a big deal for something like a Pine Snake and I recommend actually seeing what one of your snakes looks like kept in a cage that size.
For the CB-75 that UAWPrez mentioned, the listed dimensions are about:
22.5"W x 37.5"L
If they are anything like the boxes I have, which I suspect they are since they are both Iris Boxes, then the actual floor area will be:
33.5" x 18.5".
That's less area than a 40 gallon breeder, IIRC. That's about as small as I would even consider going for a Pine Snake. They are just too long and active.
Reptiletubs.com carries a tub designed for storing Xmas trees that is pretty large. I think it's 53" x 21" - that might be tough to build into a rack.
Another large box I know of is the one that is supposed to be used in the newest Vision Rack. Someone posted here that it would be like 40" x 27". From the few pictures I have seen it appears the sides don't slope much so it should be a box that provides a lot of floor area. I have no idea if this is a commercially available box or something that Scott from Vision moulded himself.
Either way, you might e-mail him and see if he'll sell you boxes by themselves.