Ah yes, the old "how big will he get" subject! One of my all time faves! I have started a thread with the exact same title in the mixed breed forum! Consider yourself lucky that you at least have a purebred dog on your hands so you know the genetics!
Here's a link to a rottweiler growth chart, just averages of course.
http://www.der-rottweiler.com/menu2_01.htmlHave fun!
Oh and my (mutt) pooch Nigel was about 50 lbs at 5 months, and he's about 80 lbs now at 8 months, I think they stay on that 10 lbs a month track for a while and then slow down around 9 months... only time will tell!!!
(To be honest, a closer look at that chart says to me that the male rottie they are charting is growing a bit too fast... just my opinion! wonder what they are feeding!!!)