We have a 8x16 and a 9x16. Both are "Kicker wagons" (ie: they have sides on them).
The 8x16 has 8' tall sides, but the floor is ~1' higher than the 9x16 which has 9' sides.
We run 8x18 kicker wagons. The extra foot in the front allows me to stack the wagon flush to the front without having to walk in front of the wagon & throw them in there. Also allows for a few extra when we need to kick them right from the baler.