He is right, for the minimum, the native system resolution is the better choice.
For the maximum (for highter system resolution), mGalaxy can only display SS with 1/4 of the screen on its interface, so on 19200*1200 resolution, the better resolution is around 800*600, and a SS with this size can exceed 1M.
Edit:
From my memory the ps2 use natively 640x480.
And the WII too.
So IDK wich one emul you are using but taking the native resolution every time is enought for me, you will never reach the maximum resolution mGalaxy can display on an HD display without loosing data.