Stamp Size Determination

If I'm not in the correct forum, please let me know, and move my post there.
How should I measure the stamp's picture size?
From the perforation? If so, from the top of the perf teeth or from the bottom?
If only the picture, do I look for the widest part of the picture, if it's not a simple rectangular image, or the most dominant width?
I'm arranging my Russia/USSR collection, and on top of that, the catalogs point out differences of half a millimeter, which is hard to determine, especially with the unclear measurement rules. Thanks to anyone who can share knowledge or experience.

When the catalog notes similar stamps have different sizes the measurement is always from edge-of-image to edge-of-image. Ignore perforations.
If you need to determine mount size then it is from tip-of-perf to tip-of-perf plus another 2-3 mm.

Thanks, Germania. What do you do if the image isn't a square?

The catalog will indicate which direction to measure. Here is an excerpt from the Michel catalog for Germany. The footnote tells you that 364X is 31.5 mm wide by 21.75 mm high. 364Y is 32.75 mm wide by 21.5 mm high. There should be something similar in your catalog for the stamp(s) in question.