When B5 or EXEC paper is loaded in the paper drawer, you can set to distinguish the size for the operation.
1.
Press 
→ [Preferences] → [Paper Settings] → [B5/EXEC Paper Selection].
→ [Preferences] → [Paper Settings] → [B5/EXEC Paper Selection].2.
Select the paper drawer → select [B5] or [EXEC] → press [OK].