Bordered and Borderless
There are two types of prints available: borderless prints, and prints with margins. This is because, for those who wish to frame their prints, it is easier to put the print in a frame if the print has a margin around it.
Borderless prints have the image all the way to the edge of the paper. For some pieces of art, this may mean that the very edges of the piece have been cropped. The sizes available have been chosen with this in mind, with the intent to keep the cropping to a minimum. Each print will thus have between three and five sizes available, depending on the piece. If a size is not offered, that is because too much of the art has been cropped, thus making that size unattractive.
Aspect Ratios
Print sizes will either be in inches or centimetres. This is because the divide between Imperial and Metric even extends to paper sizes. More specifically, the standard paper sizes in the two systems have different aspect ratios; that is, the ratio between the width and the height of the paper. For the Metric system, paper has been standardised in the A0/A1/A2/A3/A4 system where the ratio between width and height is 1 to the square root of 2 (roughly 1:1.42). This ratio has the lovely property that, when you divide the rectangle in half, the two resulting rectangles also have that same ratio. The Imperial system has a number of different aspect ratios, but one of the most common is the ratio of 1:1.5 (such as 12″x18″).
So why am I having multiple aspect ratios rather than sticking to one? Because, while most of my fluid art is painted onto A4 or A5 sized paper, I always crop the digital version to either one ratio or the other, depending on which one I feel looks better. And the art which starts off as purely digital could have any aspect ratio I feel like.
And of course, there are also squares. These are given in inches, because that is what Printful offers.