depends on how you design it, some dumps use a muli stage cylinder that has no loss in mechanical disadvantage, and then some use a short stroke cylinder and a "hoist" that extends the perceived length of the cylinder,
I would suggest to down load some broachers or info pages on hoists, and "copy" there design and use there length recommendations,
or if you plan on making your own, (I would suggest to make a template out of card board, or 1/4 ply, and then lay the cylinder on the template and work it seeing nothing bines and that is long enough or the mechanical advantages are close to what you think they should be, usually the greatest disadvantage is just as it starts to lift,
