What works for me is to play around with it:
https://learn.microsoft.com/en-us/azure/data-factory/control-flow-get-metadata-activity
see what it needs as configuration, what it returns - use debug option to run the pipeline and then check the activity output.
Some more info here
https://stackoverflow.com/questions/65152580/azure-data-factory-v2-check-file-size-for-copy-activity
Idea would be to have this check as the first step in the pipeline, it cannot, as far as I know, be a part of the trigger.