As I posted on the wedding forum, I agree that this is ridiculous. I would be laughing my head off if I got an invitation like that. (I love the post above about the sparkly special snowflake sled.)
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? If you're broke, would you ask them not to wear real jewelry or outfits that cost more than $50? (And isn't this sort of an insult to your short guests -- the implication that short = unattractive?)
That said, I agree with the posters who said that although the request is silly, inappropriate, and arguably even rude, that does not give guests a free pass to be retaliatorily rude -- i.e., to attend but deliberately ignore the request, stupid and inappropriate though it may be. You don't get to punish people for their etiquette blunders.
I'm 5'1", ALL my nice shoes have heels, and I would not enjoy complying -- but if I attended the wedding, I would, the same as you suck it up and wear a costume or tuxedo/black suit if that's what is requested even if you hate wearing costumes or black tie. The fact that those are etiquette-sanctioned requests and this is not does not change the guests' duty to be polite and comply if they attend.
It's moronic, entitled, and inconsiderate, but it's not like they're asking them to bring a freshly killed puppy. The bride needs to get over herself, but so do the guests.