Im not sure specifically how to correct this, but like the others are saying try a test cube or cykinder. You could download one, or use cura, but id sughest make it in tinkercad and import it just like you would notmally.
In theory if you designed a 20mm cube, it should be just about bang on if everything is set up right.
If your finding it isnt, check teaching tech’s website. The other guys think your axis steps may be off, so there should be a calibration you can follow to correct this.
Not all printers will use the same steps, but with how far off your measurements to actual print are, perhaps you can tell us what printer, control board (stepper motor drivers) and what your x,y,z and e steps are, and we can try a simple comparison.