I don't have a current source. However, as far as the size is concerned:
How many camera pixels per inch (resolution, ppi, etc.) that is desired an important thing. Everyone wants 600ppi. But, you need to think, how far away the viewers will be when they view the print. It might not look great from 18-24", but if they are 5 feet away, it might look great.
I know I haven't explained this very well, but I hope you get the idea. Many large prints are not meant to be hand held. Years ago, when I was contemplating a large format printer, I did see a guide to help determine viewable size based on uncropped pixel count, etc.