I agree that this is ridiculous. I would be laughing my head off if I got an invitation like that. (I'd be tempted to ask for an exemption because I am only 5'1" and don't tower over anyone even in my highest heels, just to be obnoxious. And then I would recall that I don't have any flat shoes that would go with wedding-guest clothes - mostly flipflops or sneakers - and be annoyed that I'd have to go buy something just for the one day.)
What's wrong with being short, anyway? You're short, you're short -- that's who's getting married. Why is it important to be taller than your guests? If you were ugly, would you ask your friends not to wear makeup? If you're heavy, would you ask them to wear unflattering clothes?
I'd feel the same way about instructions to wear a certain color or other overly controlling instructions. Other than for a costume party, the hosts set the formality level and then let the guests dress themselves. You want to tell them exactly what to wear, make them attendants.
An invitation like this makes the couple appear like they are more concerned with being king and queen for their !!Special Day!! than with the part about getting married.