Hello Rachel,
We used to have a blue dane bitch several years ago, who we got from Ida Wheeler at Bullaine Danes in Maidstone. It was always our understanding that the colour was a factor in the eventual size - the blues and blacks being the smallest and the brindles and fawns being the biggest.Of course it also depends on the size of the parents.I can't really remember but I think Cleo was easily fully grown in height before 1 year.
The problem with dogs (a bit like kids) is that they haven't read the 'owners insruction manual', and grow (or not!) regardless.
We were told to expect Cleo to live until she was about 9 years, but in fact she lasted till she was just short of her 14 b'day!

I hope you will be very happy with your Dane - they are such lovely gentle creatures,
Regards
Clare