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?