If you can't slide the 5th wheel ahead because the trailer hits the rear frame of the truck it's hard to load the front axle. The shorter the upper deck the less clearance you have between the truck frame and the drop. I'm with G-Man on the 11' upper.

 

I have 35,000 lbs in my dry van trailer and the fifth wheel is in the perfect position for weight distribution. I measured the distance from the king pin to the support legs of the trailer and it's ... 8 feet almost exactly. The problem is for some reason the king pin is really far from the front of this dry van trailer - close to 36 inches. 8 + 3 = 11 feet. Now an 11' deck makes sense but that leaves only 42 feet to move that expensive stepdeck freight on the 53' trailer

 

It is the upper northwest corner of California where all the redwood trees are located. I don't recall the name of any of the towns off hand. It is not far from Oregon. I think the road is windy and a longer trailer cannot easily navigate it.

 

Anything over 40' on the lower deck is a plus. You don't need more than that for most loads. There are some where I have needed the entire 42'. You have a little more flexibility with 11' on top. I keep most of my equipment on the top deck so some of that space is lost. In fact, I probably lose 1-2 feet with my equipment. If I cleared the upper deck I could haul a small car on it. Anything smaller I don't think would fit. Most things that I have hauled would work on either a 10' or 11' but there have been a few times when the entire top deck is needed in order to accommodate my equipment and freight. Clearance is also a factor. One truck doesn't have a problem clearing the stepdeck. The other needs the 5th wheel slid all the way back to easily clear. Part of the reason is probably that I have tall rubber on that truck. The down side to having a 53' over a 48' is that you will add a little weight. It hasn't made much difference in what I have hauled. As far as an 8' upper deck is concerned, it would not work for me. I think it would also be difficult to sell when you get ready.

 

You can get trailers with 18" kingpin settings. I have a lot of room under neath my trailer.