I can't remember the command line you add to the startup of HL2. Or is there a *.cfg file you can alter? (I couldn't spot any reference in the config.cfg, someone prove me wrong)
just type regedit in the run thingy and go to hkey user/ software/ valve/ source/ hl2/ settings and u will c screen width and screen height double click on one of them and change the base to decimal then type in the height of the res. mine is 1024
That seems rather over the top for a simple command line action. (Which is why i originally posted 'search' here to give a hint that you could just do a search and you'd find loads of topics with this. Evidently this wasn't liked)