If you use a lot of different pre-set sizes, you can also use my script. I'm constantly forgetting the pixel dimensions, so I made this (with the help of several people on the forums). This one is set to 16:9, but you can update the values to any ratio or entries that you like. Just update them in the array (for the pull down list display) and then in the function so it sends the correct values to the rendering engine.
The only two caveats are:
- This only works on the active renderer, it would have to be run again for the non-active renderer after switching.
- There is a limited number of selections that can be in the list at one time. You can always duplicate the script and rename it if you want a choice between different aspect ratios.