I find the britching the toughest part to measure, we have three different carts and a pair setup, that meant we ended up with four sets of britching straps. It just depends where the footman loops are positioned on the shafts or in the pair setup we go all the way to the D rings on the breast collar. But then they are probably the easiest part to order extra's from just about any supplier.
The one thing I miss in this thread is the length of the traces and breast collar combination. From single tree around chest to singletree with 8 to 10 inches of stride space behind the horse is what I understand.
Some like to have the horse further forward but IMHO that makes it that much harder to get around corners. So it not only speeds the game up but also makes a more elegant turnout. That is also one of the reasons the HyperBike is so unique, it turns on a dime.
My point here is you not only have to look at measurements of the horse but also the cart and what kind of driving you are planning to do. So you have a harness that is actually working to support all of that.
We ended up with seven different harnesses for two horses that fit the same size and I am sure we not the only ones that went through different used ones before we said "he lets do it right this time"