Summarize:

Your locket should have a frame around the area designated for the image. You want to find out the measurements of the image space. If possible, measure this with your ruler to the closest millimeter or fraction of an inch. Use a ruler or tape measure to find the size of your locket. Knowing the measurements will provide you with a reference point to use when resizing your image. If you are unable to measure your locket's image space, you can estimate it. A general estimate is about 1 millimetre (0.10cm) or so smaller than the size of your locket. It is better to guess high than aim low, because you can always trim the edges of your image later. Circular lockets may be trickier to size because they do not have straight edges. Measure across the circle horizontally to find the diameter. You can use this as your approximate width. You can estimate the height based on the top and bottom of the circular shape. It's okay if your measurements aren't exact. Get as close to an even fraction as you can, and estimate to a larger number rather than small. That way you can trim the image to fit, if you have to.
Measure the size of your locket's image space, if possible. Estimate the size of your image space, if difficult to measure. Measure the diameter instead of the width if your locket is round.