I have taken pictures and am sending them into the printers to get them blown up. I am starting with a 4 by 6 photo. And I want to a variety of sizes that are up to two feet on one side. I just cannot figure out all the sizes in between to get there. I don't want the photos to be stretched, but retain original proportions. Is there a formula I can use or anything? Basically what are the 6 or so sizes that get me up to 24 " on one side. Thanks.