Can the standalone viewer display at a chosen pixel width or height? I revisited VIEWERCMD=zoom to before asking. I'd like to add a few pixel-defined width and height entries to my image viewer context menu which already has quite a few percentage entries...
Scripting can tell you the size of the viewer window (I think it's the whole window, not just the image part of it, so you'd need to subtract for toolbars etc.).
I'm not sure if there's a good way to resize it, though, without throwing third party tools into the mix.
An AutoHotkey macro might be the way to go.
Or you could open the viewer with an image that's the size you want, and configure it to resize for the first image but then stay the same size for subsequent images, which is an option in Preferences.
Actually, the Show command has POS and SIZE arguments for this, which I forgot about.