Unanswered Parts not sized correctly after print.

I ordered the smart LCD XXL from printed solid. I printed the case for it and it would not fit. I also issues with the parts fan not fitting into the fan shrouds I printed after the upgrade to e3d v6 hot end. The case that I printed seems to be around 3.5 mm short on the X axis and a couple mm short on the Y-Axis. I don't have the exact measurements with me as I am at work. I printed off a 20x20x10 calibration cube and ended up with 19.72x19.84 x 9.85. What should I be looking at to fix the issue? The part was printed using PETG green.

I fixed the issue my stepper calibration was way off and after hours of toying with it my prints are now spot on. My test cube is now 20x20x10 on the dot.