Incorrect sizing

Sounds like you need to do and X Y axis calibration in Lightburn. Also, check these things:

1. Loose belts in the direction you have a difference in measurement.
2. Is this difference happening in all quadrants of the bed the same?
3. Make sure the laser head is 90 degrees to the bed front-back and left-right.
4. Make sure the lens is not loose inside the tube/housing.
5. Are you using an online box generator? If so, report this anomaly to the programmer for the box designer. I had a problem just a few days ago with an online box generator, report the problem, and the programmer found an error in the code an corrected it.