[Gimp-user] limiting the print size of a photo

[Glenda Guy <adnelg19 outlook com>]

Hello, I am establishing a photo website and would like to have two options for digital photo downloads:  6x4 
to 10x15 and 12x18 upwards. All my photos are taken at 300ppi.

Is there a way in GIMP where I can change the photos so a customer can download a file that will print with 
optimal results 10x15 down to 6x4 but no bigger unless they choose the 12x18 upwards file?

Thanks for your help - Glen