I'm going to use 30"x20" as an example in the options below.
Framed Prints
The measurement 30"x20" is the width and height of the glass in the picture frame (see diagram below). The size of the printed image area will be smaller to allow for the width of the mount.
Mounted Prints
If you're looking at buying a print with just a mount and no frame then the measurement 30"x20" will be the height and width of the mount, meaning it will fit into a 30"x20" frame (see diagram below). The size of the printed image will be smaller to allow for the width of the picture mount.
Print Only
For print only options 30"x20" is the size of the paper. I leave a generous border around the image to allow for a mount to be added (see diagram below). The size of the printed image will be smaller to allow for the width of a mount if you wish to add one yourself. You can also frame this option without a mount if you prefer.