When A5R or STMTR paper is loaded in the paper drawer, you can set to distinguish the size for the operation.
1.
Press 
→ [Preferences] → [Paper Settings] → [A5R/STMTR Paper Selection].
→ [Preferences] → [Paper Settings] → [A5R/STMTR Paper Selection].2.
Select the paper drawer → select [A5R] or [STMTR] → press [OK].