Smaller diameter nozzles allow for finer details but the overall print takes significantly longer. Larger diameter nozzles can be required for the higher strength filaments (anything with mixed in fibres). - 0.4mm seems to be a good general purpose option, at least for PLA. - 0.2mm can make small writing much more legible - 0.6mm is best for any of the CF or GF filaments (fibre reinforced) - 0.8mm will print quickly for larger parts. Generally higher strength.