Hey Damien,
I have a client who is getting a warning that her digital file is too low res to print a 40"x60" print. I discovered that the file size 72ppi, I read your analogy about 72ppi and 300ppi but the lab she wants to use requires 300dpi. Can I just go into image size and simply change it? When I try to do that it changes the file size from 6mb to 49 mb, is that ok?