Well, I found a way to do this even though it’s not great. In case anyone else gets in this kind of trouble, I wanted to share it here.
You first have to create your own image of some kind at the resolution you want.
You can then read that image into Codea and modify it.
And then, when you save it from Codea, it will retain the size you loaded it as.
As far as I know, this is the only way to control the exact size Codea saves images at.