The width of the horse/pony neck is just as important as the length from wither to the top of the breast (between the shoulder points). So I measure by taking a tape and wrapping it around the neck where the collar will sit, note the inches, then run the tape in a straight line from the dip just in front of the withers down to the point where the animal's breast meets the neck.
Those are the measurement you want. I keep mine on a cheat sheet in my wallet as I have 4 that wear different collar sizes.
A collar can be temporarily "stretched" open a bit to accommodate a wider neck, and also to put it over the head without hitting the eyes, but it will still migrate back to its original form. Here is how you stretch a collar:
Remove the hames first. Then using a knee pressed down against the inside center of one side to hold it firm, pull up on the other side with your hands. This will widen the collar for a few minutes - often just enough to slip it on.
Just as an aside - to put a collar on, flip it upside down so that the wide bottom is on the top and the narrow portion is on the bottom. Standing next to your horse/pony while facing in the same direction, slip the collar on with the widest part passing over the eyes. Once the collar is over the head and at the poll, gently rotate it, turning it in the same direction that the mane lays, then slide it down the neck until it comes to rest with the top of the collar in the dip where the neck meets the withers. The sides of the collar should rest along the slanted lines of the shoulder blades. The bottom of the collar should just come to the point where the neck meets the breast, and there should be just enough room for you to comfortably wrap the fingers of one hand around the bottom of the collar without pressing into the horse's neck.
Once the collar is on, the hames are put on.
Hope this helps! Best of luck in finding your new collar.
The carpenter squares are the big ones and you need two. Like a big L . Seems to work better for the newer folks than a bendy tape does. You put the two longer sides together, sometimes a spring clamp helps hold them in place or even just your hands. Makes a kind of calipers, [ that you can measure the inside of, for true size. The long sides are your sliding area to widen or narrow up the short sides. Now you have 3 sides of a true square shape and are ready to measure the horse.
Someone else holds horse still or he is tied to stand still.
You adjust the long sides so the open side is towards the animal. You have a short metal side at the top of his withers and the other short side at the bottom of his neck. Long side is on his shoulder. You slide the long sides together until the short pieces are touching him top and bottom. Pull it off him and measure from inside to inside of those short pieces on the top and bottom. That is the length you need of collar, write it down.
Then you face the animal, measure the widest part of neck for width, by again bringing those short ends in until they just touch both sides. This is usually the underside of neck. Write that measurement down as well.
Check him at the mane area in front of withers. Does neck narrow a lot at the top? Maybe he is thicker down a couple inches from the mane roots. Probably should measure there as well. Write it down.
Now you have measurements to work with for the collar seller.
Please understand that collar fit can change a lot as animal gets fit or loses condition, gains weight. What fit fine before, can sore him up now with body changes.
Another thing is hames fit on your collar. New hames are NOT fitted to a real horse, look like this, ( ) . So a welding shop, farrier with a forge, will need to heat-shape hames to your animal. Trying to bend them cold will often snap them off, leave marks! Be REALLY careful with brass, marks up easily because it is a soft metal. Horses wide at the neck bottom may need L shaped hames to be comfortable, while others are just straighter.
We have hames with buckle-in tugs, so we leave the hames on collars all the time. With our shaped hames, collars will modify with use, to fit the hames and horse. Each horse has their own collar and hames, fitted to THAT animal. For us it is very difficult to try fastening the hames onto collar, on the tall horse. GTD has ponies, so collar, then hames, may work better for leverage with her hames straps fastening. We just can't get hames straps tight.
We put our collars on upside down as well, usually are wider to go over eyes, then turn collars to correct position at the throatlatch area, before sliding them onto shoulders. Take them off the same way, upside down. Ours get a treat for each on AND off, so they DIVE into their collars, stretch to get them off. Both very helpful to us grooms! With the treat they look forward to collars, not a problem they don't like.
Hope this was helpful for measuring, not as complicated to DO as it was to write out!! Ha Ha