The size of your source frame has no impact on the size of your exported file. Go back to your original project and when you are ready to export, change the export settings to reflect the frame size you want.
Audition can do 5.1 audio as advertised. Adobe does not say Audition can do Dolby Encoding so it shouldn’t come as a shock that Audition doesn’t do Dolby encoding.
You will find some useful information in the PremiereProPedia dealing with surround sound.