There is a problem with the size of the Death Star, there doesn't appear to be any agreement on size, and it is hard to judge distances in Space anyhow.
Some references quote 120km, some 160km and some 900km. I had this problem myself when doing some new scenes for Jedi a few years back.

I'd just scale them so it 'looks right' in comparison with the other films, currently it looks like they are too large in comparison.
To that end I found this the most useful reference by far.
http://www.theforce.net/swtc/ds/#dimensions