I have no idea.but a friend of mine had a great dane lab mix and at 3yrs its barley 30inches if that. So a boxer mix might be slightly shorter. Depends on how big the dane is too. Most male danes are around 35inches so you can expect to see your pup be quite smaller than that since a boxer is a medium size dog. My guess is he will end up somewhere between 26-30inches.As for growth rate when it comes to mix breeds it is never easy to predict. You never know.
I have a 9 month old female Boxer/Great Dane mix and she weighs about 70 pounds, but height-wise she's about the same size as a full grown Boxer male. You figure that the average Boxer female weighs somewhere between 50 and 70 pounds, and the average Great Dane female weighs between 110 and 130 pounds. Puts her right around 90-100 pounds full grown at around a year and a half. She was malnourished by her previous owners (we adopted her) so that might have stunted her growth a little bit, but she's already extremely strong!