In the menu did you change the aspect ratio to 16:9? That will likely solve your problem. Without that, the highest resolution is 1280x1024. Give it a try. I just played through HL2, EP1, and EP2 in the past few weeks. Great to play through them all. HL1, Blue Shift, and Opposing Force are next.
if that doesnt work, you can FORCE your game to start up in a certain resolution to do so, do the following:
Start up steam
click my games
find counter strike source
right click on counter strike source
go to properties
under the general tab click on set launch options
type in
-w 1920 -h 1080
hit okay
This should make your game load into the resolution automatically everytime.
PS: if your computer lags a little with this resolution also add
-dxlevel 85
this will make your game run smoother : )