The four main nib sizes
Most fountain pens ship in extra fine (EF), fine (F), medium (M) or broad (B). The names refer to the diameter of the tipping material that touches the paper. EF is a near-hairline at around 0.3-0.4mm; B can be 0.8mm or wider. Japanese nibs (Pilot, Sailor, Platinum) tend to run finer than European or American ones of the same nominal size - a Japanese M often writes like a European F.
Which nib for which handwriting
Small, neat writers and people who use cheap or thin paper benefit from a finer nib. Less ink means less feathering (ink spreading into the paper fibres) and less bleed-through to the back of the page. Large, looping cursive looks its best with a medium or broad nib; the wider line catches the subtle shading that makes fountain-pen ink look alive.
The feel difference
Finer nibs tend to give more feedback - a slight scratchy-smooth sensation as the tipping meets the paper. Broader nibs glide. Neither is better; it comes down to what your hand likes. If you find most pens scratch or snag, try a Japanese medium or a Western fine.
Our starting recommendation
For most first-time buyers: a fine or medium Parker nib. It writes cleanly on everyday paper, cartridges are easy to find and refills are affordable. If your handwriting is small and dense, lean fine. If it is relaxed and open, medium. You can always add a second pen later with a different nib.