Turns out you can resize AoC's window with an ordinary resize command. For example, with HotkeyNet, you can do it like this:

Code:
<Hotkey F1>
<TargetWindow "Age of Conan">
<SetWinSize 960 800>
That automatically adjusts the resolution of the 3D part of the screen (the landscape and characters). However it doesn't adjust the size or position of the interface doodads (hotbar, map, etc.). I can't find a way to adjust the size and position of the doodads. But the results aren't too bad if you set AoC's default resolution to the smallest possible size with AoC itself before you change anything with a third-party program.

I tried removing the window frame and caption with HotkeyNet's RemoveWinFrame command:

Code:
<Hotkey F1>
<TargetWindow "Age of Conan">
<RemoveWinFrame>
This works but unfortunately it causes AoC to position interface doodads incorrectly so some get cut off and others float inside the window. It's sort of workable but imperfect.