This video explains the procedure for scanning large documents.
- Add a profile for scanning A3 sized documents
- Scan an A3 sized document folded in half
Note: This is a procedure for scanning large documents such as A3, B4, and double letter sizes.
For details, refer to the ScanSnap Help:
Scanning Documents Larger than A4 or Letter Size 
[Reference]
By using the A3 Carrier Sheet, you can automatically combine an A3 sized document folded in half.
For details, refer to the ScanSnap Help:
Scanning Documents Larger than A4 or Letter Size (with the Carrier Sheet)