This sounds bizarre to me.
It is true that in order to look the part of an eq. rider, you would have to have a horse the correct size for you and that takes up all of your leg.
And I don't see how your height and length of leg affects your ability to show hunters, as it's the horse that's being judged, not the rider. Certainly some riders dress up a hunter better than others, and there's ideal and less than ideal rider conformation, but a leggy 5'9" sounds pretty close to the ideal to me, especially if you're also slim and willowy. If you look at some of the best hunter riders in the country - John French, Martha Sifton, Liza Boyd, Peter Pletcher - they are about your height and build.
Many riders get to a certain point in hunter riding where they can't afford to be competitive at a certain level or to go to the next level, and they have to make a choice - lay down the same types of trips for the same types of ribbons over and over again, or do something else.
Is it possible that these friends/instructors are saying you're "outgrowing" hunters ability-wise? And that if you want new riding challenges you should try other disciplines? That makes more sense to me.