Gramma schreef
I am not sure if this is the correct forum for this question but there are
surely some botanists who have thoughts on this.

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One of the prime uses of botanical illustrations is to illustrate botanical
/ taxonomic works. These are written by people who will have special and
intimate knowledge of the plants described. Therefore they will know what
parts of the plants it is necessary to feature, and from what angle, perhaps
mounted in a special way. They will so instruct the artists commissioned to
make said illustrations.

So it depends on the plants portrayed (and perhaps the level to which
knowledge has advanced of the particular plant portrayed. It does happen
that later it becomes known that a feature disregarded so far by taxonomists
is critical, after all).

A common device to show scale is a scale bar (usually a line, sometimes with
short cross lines at either end) that represents, say, 1cm, 5cm or 5mm in
the real plant. If the illustration is reduced so is the scale bar.
PvR