Sizing the Girth
All horses have the same basic shape, even high withered or no withered ones.
The widest part of their body is at the widest part of the curve of their rib cage. If the D rings on the girth are above this point, you will never be able to tighten the girth enough to stabilise the saddle as you will never get enough grip.
Above this widest part, the body of the horse is sloping inwards towards the spine, so the girth has no grip.. Your saddle will slide forward and back, from side to side, if not under the belly of the horse. The worst thing is you are hurting your horse as you pull tighter and tighter.
Your horse expands its rib-cage with every breath, to allow the lungs room for expansion and this must not be restricted by a girth that is uncomfortably tight. In fact, he probably tells you he doesn't like this when he turns his head and tries to bite you when you saddle him up.
If the girth is below the widest part of the rib cage, you won't need to tighten the girth so much that it stops him breathing. The body of the horse is sloping out towards the cinch, so don't have to pull as hard to tighten the girth, as you get immediate grip.The cinch can't slide from side to side either since you are already at a widening portion of the horse's body.
You need to buy a girth that ends ABOVE the elbow, so that it does not impede the front legs as he moves, but ends below the widest part of the rib-cage.
Every horse has different conformation, and one 16hh horse may need a full size girth, whilst another needs a cob.