1.5-1.8mm. It covers 80% of DIY home projects.

Yes, but it's not ideal. Cars need 1.0-1.3mm. Furniture needs 1.5-1.8mm. If you do both often, buy two sprayers (or one with interchangeable nozzles).

Too small: Paint won't come out, or sprays very slowly, with a rough texture

Generally yes. But electric sprayers (especially HVLP) are more sensitive to paint thickness. Consider going 0.1-0.2mm larger than you would for pneumatic.

No. The paint is too thick. It won't spray, or will come out as sputters and clumps. You need to thin the paint or get a larger nozzle.