Spoiler:
Assuming you are allowing for the "status" toolbox (it's about 30 high)
Portrait: Xephyr :1 -title "L:A_N:application_ID:xephyr" -screen 600x770 -cc 4 -dpi 135 &
Landscape:
Xephyr :1 -title "L:A_N:application_ID:xephyr_O:R" -screen 800x569 -cc 4 -dpi 135 &
If you prefer your "status" toolbox removed (I need it for the space hungry audio apps)
increase the above sizes by 30 and:
wmctrl -r L:A_N:titleBar_ID:system -e '0,0,0,<width | height>,1';
Like:
wmctrl -r L:A_N:titleBar_ID:system -e '0,0,0,800,1'
and wmctrl -r L:A_N:titleBar_ID:system -e '0,0,0,800,30'
to reset...
EDIT:
Xephyr :1 -title "L:A_N:application_ID:xephyr_PC:N" -screen 600x800 -cc 4 -dpi 155 &
and
Xephyr :1 -title "L:A_N:application_ID:xephyr_O:R_PC:N" -screen 800x600 -cc 4 -dpi 155 &
Hope that helps. you can play with the dpi as you wish.