How much are you planning on putting in your trailer? Will it be a lot of relatively light but bulky stuff (like sleeping bags)? Are you going to have stuff in containers (like Rubermaid Action Packers)?
Think cubic feet (volume: length x width x
height). You don't want to go to all the work to make a custom trailer just to find out you don't have enough volume inside.
On my trailer, I knew I wanted to be able to stack 8-gallon Action Packers on top of each other. They are about 12" tall. So I needed to make sure my trailer tub was at least 24" tall. I went with 27" (from the floor of the tub to the top of the tub's lip measured with the lid open).
Don't be afraid to go a few inches taller. You will be glad you did.

