6'0" because the only 6-0 I've ever reached is my age.

I'm so sorry that you cry about it.  You did not do this intentionally-- it was an oversight.  You did what you knew to do, and no less.  Please don't beat yourself up over it any longer. No parent is perfect; once we see our kids as adults being parents, we realize all the things we should have done or nor done, and we grieve, but we can't change the past.  Be proactive now.  Do everything you can think of to help him accept, deal with, and get past his lifelong handicap.  Love him, assure him, study the problem with him and help him to find all the best advice on how to cope with it.  Don't tell him what you found out-- not just yet at least. Keep it under wraps for now and wait until you have shown him that you want to help him deal with this issue effectively and prosper as a young man despite his noticeable disadvantage.  You will know when the time comes and then you can tell him.  He will always thank you for the time you invested in helping him NOW while he is still trying to establish himself in the world.  Read all you can about it.  Read Ralph Keyes's books.  Figure out what he is going through and live it with him if you can.  

Try this on for comparison: my mother did things during her pregnancy with me that stunted my growth. She did them because she was childish, willful, and ignorant. She didn't intend to harm an unborn baby, but did foolish things that did.  When I was a tender 16-18 years old, and fretting about my lack of size, she was of no help to me at all-- she said silly, stupid things that only made it far worse. Till the day she died, she never reflected how much it hurt me and damaged my self-esteem.  Now, many tears and many years later, I am viewed by the people around me as a successful, influential, accomplished man who has appeared on TV and in newspapers a number of times because of my talents, and to all of them my height of 5'4" is of no import.  My mother had no part in helping me do that-- I owe that to my friends, my wife, and my most of all my faith in my Savior.     

Be the mom I didn't have.

Kind, supportive, thoughtful, encouraging comments are a mark of good character and healthy attitudes.  Pity that you lack those.

For Minilinebacker:  I get what you're saying.  Here's the distinction I'm making-- she wasn't purposefully trying to diss me; her comment was the usual thoughtless, sideways insult and my point is that I only shoot myself in the foot if I try to say anything, precisely because she didn't do it on purpose, and if I make a stink, then I am seen as being 'feisty' or too sensitive, or looking for a fight, etc.  That's my point, and my frustration with this kind of statement-- I have no option but silence since she wasn't picking a fight.  Another possibility is to private message her and explain that it was tactless, but again, she would think I'm insecure, and not understand at all what short guys are put through every day.

Now, when someone intentionally disses me over the height issue, I have a whole quiver of arrows to shoot back right away.  I practice retorts so that I know which one to use when.  Thankfully, it's rare that anybody disses me anymore, anyway-- only a few lunk-headed in-laws or a total stranger, and I rarely go someplace where it is likely to happen.