Okay, let me reiterate: Hi-res pack has nothing to do with screensize!
HI-res is *textures* only!! Drop your graphics options texture size down to 128, and check it out. It looks like arse, but everything is the same size on the screen. It has to do with density of pixels ON the objects, not the objects themselves.
Resampling up will lose some quality, yes.
[EDIT: Setting screen resolution to 1024x768 on a LCD with a resolution of 1280x1024 results in resampling up, just to clarify -- it scales the smaller resolution to fit the available pixels.]
I'm not so sure that lowering your resolution makes everything larger. It makes certain parts of the text window, and the radio bar larger, but I think it keeps planes and objects at the same scale. The same % of the screen, that is, where if a plane's wingspan takes up 5% of the total width of a screen, it should still only take up 5% at a lower resolution.
Lower resolutions just means you get less "dots" to make the picture up -- meaning you see LESS, but it's zoomed up to the same size.
It's been a long time since I ran at 800x800, and once I got my LCD I stopped running at 1024x768 because of quality issues (non-native resolution). However, I seem to recall that flight objects you shoot at are the same size in all of the resolutions. (*shrug*)