From what I've learned by just imaging and experimenting, image scale and FOV are the primary considerations. Object brightness (magnitude) can be accommodated by total integration time. That said, a handy phone app, Skysafari for one, has a feature that I regularly use -- 'Observe-Scope Display' in which you create/plug in your equipment specifics (focal length, pixel size, sensor size, etc. and the app generates a FOV and places it on the viewing map. You then select an object from the database and can immediately see if the target will be sufficiently sized within your specific equipment's FOV. You can then rotate this indicator to 'compose' the target and apply that rotation to your camera/scopes orientation to match.
For magnitude, after awhile you will learn what general settings/sub lengths work to produce useable images. For example, with my general terrestrial gear anything above mag 8 ish and I can start with around 1-2 hours total integration time. I wish all objects were around Mag +4! For mag 10 that may go up to double the total integration time. Most of my targets despite their birght magnitudes takes me months to image properly due to my location's seeing conditions, L.P. notwithstanding, so I just head out, re-target and acquire more data.