Sorry no one has answered you yet. Most manufacturers should have instructions on how to measure for their particular harnesses so if I was in the market for one, and I wasn’t able to tell from the store I was purchasing it at, I would verify with the manufacturer. Most harnesses in my experience are measured using neck inches. Some may give you a weight limit as well.
With that said it is true that some don’t like harnesses, myself included. I think they are too dangerous and not worth the risk at all. My bunnies are perfectly happy in the comfort of their home. I use a carrier for vet visits and that’s all I need to keep them safe and comfortable when they do have to go out of the house. Not only are harnesses risky but taking them out in public also has it’s own risks. I only mention this so you don’t feel bad if you find you can’t take your big guy out at all. A lot of rabbits don’t like harnesses to begin with.