Carpet 1.4.40, Carpet-extra 1.4.40, Minecraft Release 1.17
Below script's 'world size' argument acts as a string and gives these three suggestions (not options): "", "quoted phrase", and word.
The script is loaded as a file, not as a command, and is a .sc file, not a .txt file.
simulator_load_cellular.txt

I think this is #936, which is fixed in the upcoming version.

Until that the workaround is to not use a _ in custom argument names (or to get the development build).

yeah, you would need to define 'size' argument type, but that is technically enabled for 1.4.41, so closing. So either define it as 'size' or use .41 on 1.16.5 or wait I guess till 1.17.1