Sorry I havn't been able to post this sooner, I know your game is calling you
, Anyways here is how you change it back.
#1 Create a shortcut for CS:S on the desktop from Steam.
#2 Right Click -> Properties
#3 In the target line add,
"-width 1024 height 768" - Without the quotation marks.
(Or change it too what ever you want your RES to be.)
Make sure you include a space between the '-' and the existing text.
Changing your 'config.cfg' also does this. or you could just add the above lines to ur config.cfg