How does the size of the fence affect the distance you need?

The size of a fence significantly affects what constitutes a comfortable takeoff distance because larger, wider fences require the horse to leave the ground further from the fence base to produce the arc that clears both the height and width of the obstacle — while smaller fences can be safely jumped from a closer takeoff point that would be dangerously tight for a bigger fence. For a small cross rail or vertical, a relatively wide range of takeoff distances produces a safe jump — the horse can take off somewhat close or somewhat far without compromising safety, because the fence's modest dimensions allow flexibility in the takeoff point. As fence height increases, the range of comfortable takeoff distances narrows — a larger vertical requires a more forward takeoff point to produce the arc that carries the horse's body over the top rail, and a very close takeoff from a large vertical forces the horse to jump very steeply to clear the height from a cramped position. Oxers — spread fences with both front and back rails — add the dimension of width that further affects the ideal takeoff distance. A wide oxer requires the horse to take off further from the fence than a vertical of equivalent height because the horse must jump both across the width of the oxer and over its height, and a takeoff that would be comfortably close for a vertical becomes dangerously tight for a wide oxer of the same height. The practical implication for riders is that the comfortable distance to a large fence, particularly a wide oxer, is further forward than the comfortable distance to a small fence, and a rider who approaches a large oxer on the same pace as a small vertical will typically find the distance uncomfortably close. The relationship between fence size and appropriate takeoff distance is one of the reasons that jumping bigger fences requires more forward, more established pace than jumping smaller ones.