I don't think it's the height that bothers you, but something else. Because if height is that big of a deal to you, you wouldn't have dated him in the first place. You might have felt attracted to him before, but now you lost those feeling and you're just trying to find a reason to get out of the relationship. If you want to break-up with him do so, but don't use the height as the reason, that's just shallow and immature. At least give him the respect and tell him the truth. Now if he was an ass to you during your relationship then by all mean do whatever, but if he was a gentlemen to you than just tell him your feeling for him has change and that you no longer have romantic feeling towards him anymore. Simple as that, good luck.