I only paint horses as head and shoulder studies. This is to allow sufficient room for detail, which would otherwise be lost in painting a full body work of such a large animal.
Here's a few options to think about when ordering a horse portrait:-
1. Decide on a size
Due to the size of the animal, I find the best dimensions for horse portraits are either 16" by 12", 20" by 15" or 24" by 18". If you are considering a 24" by 18", please bear in mind that once the portrait is mounted and framed the final size will several inches bigger - you might want to check first that you will have a suitable place to display such a large work.
2. Consider the angle of the horses head
In the example opposite, "Harley" faces us. You may prefer your horse to be painted at three-quarters on, as in the examples below (both these approaches allow us to see both eyes of the subject), or at right angles to us (see this example of "Dignity").