Starting with the Photo mode, technically you are constrained to four sizes, as shown in the following screenshot:
What if you need a different image size? The only option is to render a bigger image and then resize it.
The only downside is that if you want to do some color correction and other adjustments, you only can export an 8 bit image (which doesn't give you much room for tweaking colors) and use these formats:
Joint Photographic Experts Group:
*.jpg
Bitmap:
*.bmp
DirectDraw Surface:
*.dds
Portable Network Graphics:
*.png
Truevision Graphics Adapter:
*.tga
As mentioned before, this is one option to export a still image and although the Photo mode provides a friendly interface, there is a better option that makes all the processes easier.