I would like to change the screen resolution in Age of Empires 1. Is it possible to do so from outside the game?
Not officially no, but you can use upatch-hd!
There is no official way of changing the resolution outside the game with Age of Empires, but you can use upatch.
What upatch-hd is:
UPatch is unofficial update (user patch) for Age of Empires: The Rise of Rome Expansion (only). A solid improvement over the original game, UPatch features custom game resolutions (up to 1920x1200), over 150 bug fixes, new gameplay options and other enhancements. UPatch will update your game to version 1.1. Included languages are German, French, Spanish, Portuguese and Italian (you can select language when installing).
There is a utility on SourceForge called DxWnd:
It allows you to run pretty much any game in windowed mode, including Age of Empires.
I use this program to play the game so know it works.
It works by intercepting system calls regarding full-screen mode and translates them into a resizeable window.