Samsung ML--6100 - Manual (Page 23)

5. Make sure that the stack of paper does not exceed the line indicated
by mark on the left inside wall of the cassette.
2. 6 Printer Setup
6. Pinch side guide as shown below to adjust it to proper paper width.
The paper cassette is preset to load A4 size paper. If you want to load
letter or legal size paper, adjust the cassette for the paper you want to
load. For details on how to adjust the paper cassette, see ‘How to Adjust
Paper Guides’ on page 3.3.
7. Insert the cassette back into the printer.