It all comes down to FoV and image scale, both are determined by scope FL and camera's sensor size and pixel size. Determine what targets you want to shoot and the best scope FL for the target depends on the sensor size. If it is a small target vs. your sensor's FoV, then image scale comes into play to give the resolution needed for tight crops.
Check out this spreadsheet that I created with many targets and the optimal FL depending on the camera (I have several popular camera sizes in different columns) and the amount of FoV margin around the tightest dimension (I typically use 20% margin).
The spreadsheet also estimates the best time of year for the targets and their meridian altitude.
Astro Targets.xlsx 24.5KB
From this you can get an idea regarding good FL and camera sizes. You will find that you will want multiple scopes/cameras to shoot all the targets you want. For me, I use my Esprit 100 at 413mm for the majority of medium targets, EdgeHD 925 at f/7 for the small targets, and camera lenses for the very big targets (Canon EF300mm f/2.8L, 200mm f/2.8L, Samyang 135mm f/2). My current camera is the ASI183mm-Pro, which is great for the Esprit and camera lenses, but I plan on getting an ASI2600 mono (not out yet) for my EdgeHD, as it needs larger pixels than the 183.