How do I open chromium full screen?
As many of chromium's command-line options are “added and removed at the whim of the developers”, man chromium-browser only documents a few “relatively stable flags.”
A developer How-to however links to this list of options, where among many others you find:
--start-fullscreen
Specifies if the browser should start in fullscreen mode,
like if the user had pressed F11 right after startup.
A quick test with Chromium 61.0.3163.100 reveals that this option works at least with this version:
chromium-browser --start-fullscreen