Measuring a picture
You may want to measure vertically and horizontally as shown on left.

Alternatively, it might be a good idea to get some paper and cut out an
oval shape based on the standard sizes and hold it over the picture to
see how it looks. Don't forget to subtract a little for the lip, which is
approximately 5/16 inch plus or minus.

Be careful not to damage your picture! Also, it would be a big mistake
to cut into your picture until you actually receive your frame.