I"m assuming you are riding dressage, right?
I think your instructor is WRONG to think that giving your stirrups that are way too long will help you achieve a long leg. Better to take them away, totally.
No, riding with stirrups too long is one of the MOST common things that amature dressage riders do to mess up their riding quality.
I remember reading a really great article, years back, in "Dressage Today" (I used to subscribe to that magazine), by rider Heather Blitz, where she described exactly what happens when you have stirrups that are too long. She described it in detail, and I cannot remember all of it, but basically because you are 'fishing' for your stirrups, you will often roll forward onto your 'fork', which puts you in a 'weak' position where you are easily pulled out of balance by your horse. It is harder for you to get your horse in front of your leg, and you may end up pulling downward on the reins to balance yourself.
(oh , how I wish I could find that article! Ms. Blitz wrote it so beautifully!)
Anyway, your leg should have a roughly 90 to 95 degree angle of opening behind the knee. With very long stirrups, that opening becomes way too open. You leg becomes of no help to you, and in fact, becomes a literal 'drag' on your body as you reach with your toe for your stirrup.
A long leg has more to do with how the energy and weight go down into a supporting leg, than how far down the horse's barrel your leg reaches.