How can I tell inkscape how "big" is 1:1 zoom?... It's not the same relation inkscape units/pixels on every screen, of course it depends on screen size and resolution, so there must be a way to define that, but I can't find it.

You don't have it in official 0.46, but you can download one of the latest development builds (the latest ones are safe). There you can find under Inkscape Preferences > Interface a slider where you can adjust zoom.