That's what i usually do when buying diamonds on ebay with almost no description! They almost always have ring size, so I'd ask for a profile pic and calculate diamond diameter based on inner ring diameter. Works like a charm and a few times the diamonds were much larger than they said!ForteKitty has the right idea. I took a slightly different approach.
I measured the two pearls just as they appear in the enlarged photo. The enlarged image measures 25mm for the larger pearl and 23mm for the smaller. Since we don't know what size either pearl really is, I calculated two scenarios.
If the smaller pearl is really 13mm, then the larger pearl has to be 14.13mm.
I made a ratio to solve for X, where X is the size of the larger pearl:
25 is to 23 as X is to 13 (Also written as 25:23 as X:13, or 25/23 = X/13)
23X = (25x13)
23X = 325
X = 325 ÷23
X = 14.13
On the other hand, if the if the larger pearl is really 13.5mm, then the smaller pearl has to be 12.42mm.
I made a ratio to solve for X, where X is the size of the smaller pearl:
25 is to 23 as 13.5 is to X (Also written as 25:23 as 13.5:X, or 25/23 = 13.5/X)
25X = (23 x 13.5)
25X = 310.5
X = 310.5 ÷25
X = 12.42
Either way the difference is diameter is over 1mm.
It's also possible neither pearl is the size described.
An object of known size in the photo would help, such as a coin we know the diameter of, or a ruler.
A 13.5mm pearl is 12% larger than a 13mm pearl.
Same thing’s happening with these pearls. The white pearl measures bigger in pics both because it actually IS bigger, and also because the plane of its diameter measurement is closer to the camera lens...
They do not throw away the ones that are not exactly 13 or 13.5mm so I'm guessing that they are binned in 0.5mm "buckets." In which case you could have a 13.74mm "13.5mm" pearl and a 12.76mm "13mm" pearl -- and that is an 8% difference in diameter.
If they are on a flat surface and the imaging plane is parallel, then the relative relationship should be preserved -- unless there is a huge difference in diameter. The quarter-mm (250 microns!) difference in distance of the pearl diameter from the camera is not going to do much of anything to that photo taken from a foot away -- it's < 1/1,000th of the distance. I don't think my same-height son will look taller than me if he is 1/10th of an inch closer to the camera in our family portrait.