The options menu has not yet been implemented. For now, just edit the config file saved after quitting the game. In Linux, this is $HOME/.redpixelrc. In Windows, the redpixel.cfg located in the program's directory.
Use the screen-w and screen-h variables to set the screen size. I recommend using higher resolutions if possible as many of the sprites are drawn in double size. This will reduces blurring from stretching the textures.
The sound-volume variable should range from 0 to 255.
Reverse-stereo reverses your left and right speakers.
The particles variable sets the amount of particles (percentage) to spawn. It should range from 1 to 1000.
If you video card does not support the GL_FUNC_REVERSE_SUBTRACT blender function, you will know. You should enable disable-reverse-subtract. However, your screen will be much brighter than usual.