I was speaking with a Dressage instructor from Scotland recently and she gave me a little visualization tip when sizing a horse up to see whether or not they are proportionate.
It goes as follows:
The size of the head from poll to the very end of the mouth is your base measurement
The neck from point of wither to point of the poll should be approximately 1.5x the length of the head (ideally it is 1.75x the length of the head).
The point of the wither to the point of the shoulder should be 1x the length of the head
The point of the hip to the point of the back of the hip should be 1.25x the length of the head
The saddle position should be 1x the length of the head
The point of the hock to the floor should be 1x the length of the head
I used a lead rope to measure these things and found out that our mare Ana Bella was on the mark for these measurements; however, Zorriola was a bit short in the hip and shoulder.