On their website they say the following
0.25 mm nozzle: 150 to 60 micron
0.40 mm nozzle: 200 to 20 micron
0.60 mm nozzle: 400 to 20 micron
0.80 mm nozzle: 600 to 20 micron
That confuses me. Why can I go down to 20 micron with the 0.40, 0.60 and 0.80 nozzle but only down to 60 micron with the much smaller 0.25 nozzle? Is that a typo and should say 6 micron?
You need a certain minimum flow rate to achieve consistent extrusion. Flow rate is the product of print speed, extrusion width (proportional to nozzle size) and print speed. If you use a very small nozzle and very low layer height, you'd need a very high printing speed to achieve a reasonable flow rate. Therefore, it's quite possible this is not a mistake and intentional.
Keep in mind that Ultimaker uses 2.85mm filament. With a 0.3mm extrusion width, 0.02mm layer height and 60mm/s print speed, you would need a feedrate of 0.06mm/s into your extruder. The extruder might not be able to develop enough force on the filament at such a low speed (which, owing to the small nozzle size, requires a relatively large amount of force).
The ultimaker can not print 6 micron layers since the smallest increment the Z-axis can move in is 5 microns. 6 microns is not a multiple of that.
In addition of Tom van der Zanden's answer, when the filament moves too slowly through the heated part of the printhead it is very likeley to clog.
I have had this multiple times on my UM1+, most of the time resulting from a heated printhead with no extrusion (before or after prints). So you need to be sure to have a minimum of filament extrusion happening, wich is most likeley not the case when having 0.25mm * 20 micron instead of 0.60mm * 20micron, for example.
You could of course make the printhead move faster, resulting in a higher extrusion, but that will lower the print quality again.
But let me tell you this: Always look out for high extrusion, removing clogs is a real pain!
