I was solving this use case with Heather via email but in case anyone else is interested in a solution, here it is.
In case you don't control the transfer process, you need to setup a jobflow that will periodically check the size of the file and compare it to the size the file had during the last iteration. You can do this using Loop
components. Nevertheless, this process is prone to false positive result due to possible network errors or other aspects that may significantly slow the transfer down. Therefore, it is important to set the value of the delay in the Sleep component high enough. If the size of the file doesn't change after this delay, the file is considered fully transferred. Attached is an example that should give you better idea.