In L4D's launch parameters, use the flags '-w xxx' and '-h xxx', with the x's replaced with # pixels, to set the width and height of the resolution to render at.
E.g. on a 1080p monitor, your launch parameters would look like:
-w 1920 -h 1080
or much lower,
-w 800 -h 600
You can also set windowed or fullscreen in the parameters with '-full' or '-windowed', as well as specify -noborder eg:
-w 1024 -h 768 -windowed -noborder
would load the game up at 1024x768 in windowed mode with no border. The game remembers the settings after removing the parameters, unless you change the settings in the in-game menu.
Find a full list of command line options here: http://developer.valvesoftware.com/w...d_Line_Options
Also, you access the parameters by right-clicking L4D2 in Steam library, going to Properties, then 'Set Launch options...'
Hope that helps you.