Ender-3 V3 SE Dimensional Accuracy

Had my Ender-3 a couple of months, everything going pretty well . I can work out most things but I’m a bit worried about the dimensions of prints.
Printed a credit card holder and it is to narrow for cards?
Printed a desk organiser with slots for USB drives. The slots are to narrow for USB drive?
I have seen calibration cubes for download, I guess I will have to buy vernier calipers.
If I find something wrong how do I adjust?
For my first big project I am planning print which has large number of interconnecting pieces. They must fit.

Shell > wall ordering> inside to outside should give you prints that are at the bottom end of tolerance and should allow things to fit. Definitely look at verniers. I think my calibration cubes came out fine in X and Y just a smidge under on Z

This is likely a too high flow rate. Calibrate the flow rate. Usually you don’t need to touch steps/mm for these printers, these are known values that don’t change.