Image_Texture UV Mapping

I’m trying to get an image to map on to a cylinder, but I don’t know what size to make the image so that it fits.
I have uploaded an image of a cylinder with stars on it in a particular pattern, which is what I would like to do with the model I have. I also uploaded an image of the model. Included in the image is a cored out cylinder which I intend to map on and then slide it over the actual model. The model is from a 3d modeling software so the mesh is a mess. If anyone has a better idea on how to do this I would like to hear about it. At the very least can someone please explain how you size images for models so that the image and model scale correctly.