This is a Unity editor script that takes captures of GameObjects in a given directory.
Place the following script inside your editor script folder in your unity project assets and open the prefab screenshot window from the tools menu. Provide the directory of the objects to caputure and an output directory for images. Adjust the camera size to whatever is needed then.
This script is fairly barebones but I may add in support for it to run a capture on build and an automatic camera size calculation.