While I don't have the Arkham Nightwing, I can tell you that the Robin is at least a head taller than the Movie Master Batman. Mattel has a number of different 6" scales... Movie Masters is supposed to be the "realistic" 6" scale and is the smallest. DCUC is what they refer to as a "heroic" scale with 6 3/8" roughly equaling 6 feet (this is the middle scale). The Arkham City figures are the largest with Batman being roughly 6 1/2"-6 5/8" tall. If you want to keep going, Masters of the Universe Classics is also considered a 6" scale but those figures are much closer to 7" and the WWE figures are (I believe) actually refered to as being a 7" scale. WWE and MOTUC are very close in height but the build of the characters don't really match.
EDIT: How do I delete a post?? I didn't mean for the double post...my computer's been wonky for awhile, sorry about that.
Not sure if everyone has it or just the MODS but above the EDIT/QUOTE buttons in your post you should see one or two small boxes for your own posts, one of them is an "X" which is a delete button. If those aren't available to you, if you go into edit mode at the top there is a check box you can tick (says "[Once deleted the post cannot be recovered]" next to it) and then submit your edit and it should delete the post (not sure if it will work if someone has already replied, though).