Paper size mismatch between LRC and printer

Hi,
anyone knows why I have a paper size mismatch between LRC and printer driver?
The printer setup is A4, no doubts about it.
For LRC is Letter ...

A4 for the printer's driver  - Letter for LRC - the entire image is previewed and printed
A5 for the printer's driver - Letter for LRC - only the first quarter og the image is previewed and printed

Why is this happening and - more important - how can I reset it .. ?
Thanks, Andrea