Hello Bortig,
it's not a stupid question, but a bit hard to answer as everybody has his own preference...
A smaller nozzle (0.2mm) will spray a bit finner lines than a larger 0.4mm, so for small detail work the 0.2 would be more useful,
But a 0.4mm sprays a bit wider and would be handy when you're spraying larger surfaces, like primer, masked stuff etc...
The 0.4mm should also spray fine lines if you spray very close to the surface (remove the screwed-on-tip-cap for better results, but don't damage your needle)
I have a 0.3mm nozzle, sometimes I wish I had a smaller one but in general it does every job just fine.
Cheers and kind regards,
Steven