Using xinit you can start applications without even starting a window manager. There is a sample in Arch Linxux wiki, but you may notice that Chromium does not fill full screen.

With the example below a specific site is started in kiosk mode taking whole screen. Change the size based on your display.

exec chromium --app=https://example.com --kiosk --window-position=0,0 --window-size=1366,768