The client window should be resizable to any size, with the graphics area scaling appropriately (preserving the current field of view, of course). It would be fine if this worked only in the hardware renderer.
If anyone took on this project, please also consider letting the sidebar (hps, minimap, inventory/skills) be customizable in width, with the minimap scaling in height accordingly.
Yes, please. It's very annoying to go full screen and have it not use the right quarter of the screen and yet it sticks to the upper left hand side of the screen (if I recall correctly). I think it would be okay if it couldn't go completely full screen or to all sizes as long as it doesn't fix it's position immovably if it can't fill the whole screen.