BILL– The answer to this question hinges on a couple of factors. First of all—how deep into a corner can you ride? If you ride a deep corner that causes your horse to lose the bend, to stiffen, to hollow, or to shorten his stride, then you shouldn’t ride that deeply. A more shallow corner that lets you maintain all those qualities is always better than a deep one which doesn’t work.
That said, at each level of competition there is a specific expectation as to what sort of corner ought to be able to be ridden flawlessly. In principle, in the lower levels a corner only needs to be as deep as the radius of the smallest figure called for in the body of the test. In Training Level, horses are asked to turn from the long side of the arena onto the centerline. That’s a ten meter diameter or a five meter radius, which is as deep a corner as you need to make through the rest of the test. Generally speaking, in the slower paces it’s easier to make a deeper corner. If in the canter you fudge a little bit, as long as there are still a few straight strides on the short side, you are not apt to get into trouble with the judge.
As you move up through the levels, you are expected to be able to make a deeper corner with the horse staying in balance. Even in the FEI levels, a three meter radius is about as small as you need to go. That means, however, that unlike in Training Level where a turn down the centerline is essentially half of a ten meter circle, in the upper levels it’s really made up of a corner, a few straight steps, and another corner to get you onto the centerline. If this seems implausible, bear in mind that in Europe it is much more common to ride smaller circles then we do here. To many American riders an eight meter circle seems like a big deal. The classic volte as it’s ridden in Europe is six meters, and it’s not unheard of to ride some which are even smaller.
Remember the point is not to arbitrarily decide on the size of the arc and make your horse perform it, but rather to train him to be balanced, engaged, and obedient enough that the figure you ride can be done with ease.