Hi, I'm Bhavik. Welcome to JustAnswer. I'm reviewing your question now, and will reply back ASAP.
It seems like you may not be getting option to scan legal size document to resolve this you need to do following :-
1) Installed the Brother's drivers/software.
2) At the Start Menu, run the Scanner Utility (found in Brother\...\Scanner Settings\..).
3) Set it to scan Legal.
4) Restart Windows
Windows Fax/Scan will show legal size options for scanning