This might seem like a weird question but I'm looking at harnesses online and I can't really find any information on the internet on chest size. I'm thinking a male dog so probably more on the larger scale?
If you are considering an adult you just have to measure him. My female Aussie is large for the breed and has a 30 inch chest size and 60 pounds. She is almost as big the average female rough Collie. Not as tall but stockier. Mine has a very heavy coat especially in the winter so you will probably need a couple harnesses. We have three plus a backpack. A lady that trains with us has three for her Rough Collie too.
I Use the backpack mostly for exercise. Dogs aren't made for carrying heavy loads so 10% of their weight is max. Work up to it, it's hard on their legs. I use water bottles for weight. Then we stop and have a break, using up some water and weight.
Ours is Ruffwear brand.