Is there a way to set a comp viewer to "Fit' using a script?

The viewer object does not expose such a property and as fas as I know there is not even a menu command for this. Hence, even Mylenium's idea for a workaround won't work unfortunately, I think.

You can use the respective executeMenuCommand() functions. Not sure how much sense this makes in your example, though, since you would need to ensure that the correct items are selected and the correct panel active. Creating the script is probably more convoluted than just doing it directly.

Mylenium