The only way to accurately know the correct size thimble for you is to try one on. Since this is not possible for anyone outside of the cities with shops in which they are stocked physically, I have provided a free download of some 3D files which are replicas of the metal thimbles. They are in a format which any 3D slicer software can interpret (.stl), and owing to their shape, are very easy to print. I encourage you to look for local communities or businesses who can help you print them (some libraries and creative spaces have 3D printers too). You do not need to print every size, just the 3 to 5 that you think are closest to your finger.

Another thing to note is that the ambient temperature can change your size around ±0.5mm. Please try to ascertain your size in a room temperature setting or at the usual temperature of your workspace. In the future you may find that you want a larger size for summer and a smaller one for winter.

Only about 2-3 mm of your fingertip should protrude from the top of the thimble, and the sides should feel firm without feeling tight. If there is no pressure on your finger, the additional weight of the metal thimble will cause it to slip off while working.

If you use a round thimble or have tried one of my first-generation thimbles, the diameter of the circle will most likely still be the correct size for the elliptically shaped thimbles. *

The first-generation thimbles used a letter system, as the sizing increment was inconsistent and varied slightly between thimbles of the same size. They are not all identical to the new sizes, but they may help you to figure out your size in the new version:
A - 12.0mm, B - 12.5mm, C - 13.0mm, D - 13.5mm, E - 14.0mm, F - 14.6mm, G - 15.2mm, H - 15.6mm, I - 16.1mm, J - 16.7mm, K - 17.3mm, L - 17.7mm, M - 18.3mm, N - 18.8mm, O - 19.3mm

In the future I intend to expand the size range to accommodate more people.

* Although please note that the profile shape of a thimble varies between brands, and this has an impact on how they fit the finger.