if the results are to be held in an excel file then perhaps it will be easier to just create an excel document. have a sheet named 'input', someone just updates the cell 1, 1 with the path to the directory in question, and perhaps cell 1, 2 with the name of the use? have a little button below it, your little button runs on and runs a vba macro which works out the size on the disk that the user is using in the folder.
you need to clarify:
+ you say 'disk space a user is using on a file share', are you meaning that you want to search for files which the username in question 'owns'?
+ are you meaning that you want a recursive folder search