So you have a photo or artwork you want to frame and aren't sure what size frame you need? Firstly, it depends on the type of frame you want.
I you are looking at a traditional wood frame, you would typically order the size of your artwork including any matte you plan to use with it. Measure the exterior dimension of the art including any matte you may be using. Note that the actual opening for your picture in the frame will typically be 1/2" smaller than your artwork. This is so the frame has a 1/4" lip on each side that prevents your artwork from falling through. If the opening was the same size as your artwork, there would be nothing to prevent it from falling out. So for example, if you are ordering a frame that is 2" wide and your artwork is 12x20", the final exterior frame dimensions will be 16x24 with an opening 11 1/2 x 19 1/2"
For the more modern floating frameless acrylic frames using standoffs, you need to make sure you understand what the seller needs to make your frame. In the case of IonAcrylics.com, it is requested you order your artwork size and they will automatically add a 2" border (total 4") as you need this border space for it to look aesthetically pleasing but more importantly, you need space for the corner hardware. If your borders are too small on a floating frameless frame, the corner standoff could cover the corners of your picture or even pierce through the picture.
When ordering custom cut to order frames, be sure you understand the ordering requirements, measure three times, and ask questions.