A horse's back is built like a suspension bridge. Like that suspension bridge when we add weight to the horse's back it "sags"... and like the bridge in order for it not to "break" we need to make sure it has strong anchor points in order to have a strong back.
The first place to look is where the wither blends into the spine. You want to see a clear wither, not too high, not too low, you want it to blend smoothly into the back. Having a high wither often results in getting a "shelf" behind it -which will put some strain on the vertabrae, but also make finding a good saddle fit difficult - if your saddle doesn't fit your horse is going to be far less likely to work properly... and that's when injuries happen.
We're going to draw a line from the top of the wither to the horse's LS joint (just ahead of the croup). This is our back length line.
We're also going to make a short, verticle line at the last rib of the ribcage - and make sure that line intersects with the body line. Then one more line from the top of wither to that small verticle line you just made. This is the ribcage length line.
The length of these lines will be our back length, and our ribcage length.
The length of a horse's back is of less importance to us than the strength of that back - it's very possible to have a long back which is also strong, just as a short back can also be weak.
Now, while length of back isn't "primary" concern for us, we don't want too long of a back, or it will through the balance of the horse out too. Generally, we want a back no more than 50% of the body length in order to maintain that square horse.
The ribcage length is going to help determine the length, therefore strength, of loin. We want to see the ribcage length being 98% or more of the back length.
To go with Loin length... we're going to look at loin girth depth. Loin girth is the area from the loin to the bottom of the horse's flank - the deeper (longer) that space is, the stronger the loin is, because there's more horse there to give it support.
Another thing we're going to look at is the position of the LS joint. This joint is the rear anchor to the horse's back, and we find it by locating the soft, squishy spot on the horse's spine (right on top) just ahead of the point of croup. In a perfect world the LS would be placed ahead of the hips... but a good placement would be even with the hips... and it get weaker the further back we go. The placement of the LS is not only a factor in the strength of the back, but also in the ability the horse has to round out and collect. A horse with an LS placement too far back will never reach levels of "sit" that a horse with one further ahead of the hips will be.
We're going to then put the whole back together as a whole picture of the horse's back. What can you tell me about your horse's back?