I have attached some screen shots to show what 10x13 should looks like in the set up on A3 paper: -
The custom size, and the queue tab which shows the precise printed size sent to the driver by QU.
Another possibility is that you might have a border turned on if the print is too small.
As Terry said though, need to know which printer, what size paper, and what are the results you are getting.
Thanks, and welcome.
Fred