I own a level 3 and level 4 shredder (of course!) and above is a example of particle sizes.
To the left is level 4 (micro cut) and to the right is level 3 (cross cut).
Micro cut is about 4-5 times smaller particle size.
DIN 32757-1 is what shredders are rated in security wise.
Level 1-2 is strip cut, level 3 is cross cut and level 4 is micro cut.
There are two other levels plus level seven which is a non official extension depending on who you ask.