Pretty easy- you need to work out what the weight and wheelbase is of the largest item you will want to load, and then use their charts to check what model you need. Obviously the longer the ramp the heavier it needs to be so check carefully with a bit of wood first to make sure you are happy with the ramp angle of a particular length.
I wouldn't go less than 10' for loading into a Transit type vehicle, and even that is hairy if you don't work out how you are going to approach it. For instance, you can tilt the bed to reduct the breakover height for light things like a ride-on- just drive them straight up. However, something heavier like a micro digger could cause serious damage if you attempt to drive it onto a tipped body. So for the micro digger, I track it up forwards and 'ride the bump'- however, I find it much safer when reversing off to just stand on the blade and carefully feather it over the breakover point. With the Sherpa, it's very twitchy and you also want to keep the weight to the front, so don't stand on the platform as you load or unload (unless you have a reasonable weight in the bucket for some reason). Folowing me so far? 🤣
I got lucky and picked up a set of 12' heavier alloy ramps from the same manufacturer locally for cheap- however I tend to use the lighter 10' ramps as they fit in the bed of the Iveco no trouble. But the 12' are far easier to use safely, so if you are of a cautious/nervous disposition you might want to sacrifice a bit of convenience for safety!
Whatever you get, make sure to drill your tipper body to accept the pins supplied. I have one hole on the left, and then three on the right to accomodate differing wheelbases, on both my tipper trucks, and keep a set of pins in each. Loading something, especially tracked, without pins is a recipe for death and disaster.
Photos attached of the spec stickers for both my ramps, both made by the same company but different dealers.