Canon FAX L1000 Service Manual page 383

Lift the stopper in the center of the paper size rear guide slightly,
and slide it to the mark indicating the desired paper size.
To change the paper size for Paper Cassette 2 (option)
To adjust the paper size side guides, release them by pinching
the stopper on the left guide and slide them inwards or outwards
according to the paper size you are loading.
1
2
5-25
FAX-L1000 Chapter 5: Appendix
LGL
A4
LTR
WHEN YOU SET TO LETTER SIZE.