I would like to know in the following case which is a better option:
In the PHP script, if the $fileSize variable is larger than 100, I stop the script;
Since you are using exit and return within the global scope (not inside a function), then the behavior is almost the same.
The difference in this case will appear if your file is called through include() or require(). exit will terminate the program, while return will take the control back to the calling script (where include or require was called).