Create a new composition with your new dimensions. Drag your old composition into the new composition to nest it.
Since you're new to After Effects, I very strongly recommend that you go through the basic learning materials first:
Doing so will prevent a lot of frustration.
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After Effects will mix and match any size and pixel aspect ratio footage without problems as long as you have all footage items interpreted properly. It would help us a lot if we knew more about your composition. You should always use the presets for comp size and you should never change the PAR that a preset gives you. Distortion in non square standard comps is removed when you select PAR correction in preview.
If you don't understand any of what I'm talking about then you must read and understand the help materials on rendering, composition setings and pixel aspect ratios. To keep your life simple always work with a square pixel preset then render using a pre-set for your desired delivery method.
Oh, wow. Thank you, Todd! That was just ridiculously simple. Knowing this will make my life so much easier!
Todd & Rick, thank you. I will read your suggested materials. Good ideas!