I didn't understand the commit charge and texture buffer settings either. I have no idea were such options are located???   

Anyway, I got your image to render at 40"x30" at 300dpi by deleting the infinite plane in your scene.  I'm guessing that there is something about your shader that's overflowing the memory. ?  Try simplifing the shader or pick a different one all together. 

Irene 

Irene-