Can you get Fullscreen Mode in sketchup online if so what do you do .
Using Chrome ( I guess, most of the browsers):
Windows, Chromebook:
on : F11
off : F11 or hold Esc
On Mac - not sure if it is toggle :
Cmd+Ctrl +F
or perhaps
Cmd+Shift+F
Just F11 key is the standard in windows.
If you have a keyboard with shared functions, you might need to press the FN or function Key at the same time - it very much depends on who makes your computer.
Also, you can add it as a web-app which is essentially a full screen version.
It depends on which we browser you are using
In Edge
in Chrome “Install”
This will even install a shortcut on your desktop or taskbar in some cases.