I haven't tested, but the grid_mode should work, all you need to do is correct the underscore typo:
set grid_mode, 1 # without _
set grid_slot, 2, object2
set grid_slot, 1, object1
Picture size and quality is not controlled by viewport unless you are making a movie, in that case set viewport, ray_trace_frames and movie_quality:
#Then use either:
cmd.movie.produce('example.mpg',mode='ray', preserve=0, quality=100)
I your case, however, a single image is best made using png:
cmd.png('example.png', '15cm', '10cm', dpi=300, ray=1)
this will give you a 15x10 cm image at 300 dpi, regardless of viewport.
About starting in fullscreen:
using the command line option '-e' should make pymol start in fullscreen
this will probably not work with '-c', but you can open your script from another shortcut.
Hope this helps,