Have a look at CCDCalc.http://www.newastro.com/book_new/camera_app.html
This will guide you on the two variables you need to consider.
The pixel plate scale and the overall field of view.
The choice of plate scale is heavily influenced by your seeing conditions. If you had excellent seeing at say 1 arc sec, then a plate scale of around 0.4 arc sec/ pixel will gather as much detail as available.
The FOV should be large enough to cover the more interesting objects you want to image.
It's a compromise........