Hello
All of the print sizes our size generates for your own images are custom to that source file.
We control the long side of your image in 2" increments starting at 8"x XXX. And from there, you just want to use a multiple of 100ppi. So in the above example, using an image that is 4000 pixels by 3000 pixels, we show you sizes up to a maximum of 40" x 30".
The only limits we have are that one of your dimensions is going to be forced to fit the following list:
8″
10″
12″
14″
16″
20″
24″
32″
36″
48″
The other dimension will be scaled proportionally to maintain the aspect ratio of your image. Our largest print is 48" x 108" so if a square, for instance, you could only have a print made 48" x 48"