Why can't it be rendered as a .mp4 without After Effects resizing it?
It can. You just need to set proper settings in the MP4 output module itself - like upping the MPEG Level to 5.1 to remove format constraints.
Mylenium - Thanks for your really quick reply.
I don't see an option to get the MPEG level up to 5.1 which is probably an oversight by me.
In the "Output Module Settings" dialog box
on the "Main Options" tab I select "Format" as "MPEG4"
If I click the "Format Options..." button the Level dropdown box only goes up to Level 5.
I can't help on getting After Effects to render the format you want because my machine is in the middle of a render and I can't test, but I can make a good suggestion: don't use AE to make an MPEG video. Render an intermediate file out of AE and then use the Adobe Media Encoder to do your final compression. AE can't do multipass encoding (among other things) that make MP4's better quality at smaller file sizes.
I made a small breakthrough.
I changed the format to "H.264" rather than "MPEG" and the warning message disappeared as required.
So the question becomes:
"Both of these formats output an .mp4 file, so why does the MPEG format not allow me to create a video at 1024x768 but the H.264 format will do this just fine?"