Image Size vs. Print Size
This refers to the actual size of the printed image - the dimensions that will be ink on paper. This does not include a border.
The total dimensions of the print, including borders!! This is the size that you are charged for (except canvases, read below).
What this all means is that if you want a 20x24" image with 2" borders all the way around, yor order would look like this:
The end print size is 4" larger in both directions due to that 2" border all the way around. You would be charged for the 24 x 28" print size, and that would fall into the catagory of 24 x 30" prints (odd sizes are charged the nearest, highest print size).
If your print is an odd size over 6 ft2, then it is a custom size that will be calculated.
Canvas prints are likely to be stretched. For this reason, it is a given that you will want a 1-2" border all the way around the image. This is included into the price. Larger borders, or overlapping (printed) borders are a bit extra, and can be calculated.