I think this is a Mac-specific issue, and probably unrelated to the forum software.
In Chrome on Windows 10, the "Choose File" button brings up a normal Windows File Explorer window, which works (including changing image preview size) just like any other File Explorer window.
On a Mac, it appears that it is bringing up the standard Finder window (essentially doing the same thing as in Windows - letting the computer's operating system utility handle this task), so only a Mac user can likely help with how to control this Finder window... or why it isn't controlled the same way as any other Finder window. I use Macs, but only when necessary.
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