Finding the right nib size for your writing is a very personal choice - but some quick advice is, if you write very small, choose a finer nib, like Fine or Extra Fine.  Larger writing can accomodate the wider nibs sizes. The vast majority of people will happily use a Medium nib so if you're unsure, this is the one to go for.  But if you're a writing enthusiast who like to add flourishes to your writing, you might like the extra thickness of a Bold nib. Our Fountain Pen Nib Advice guide goes into a lot more detail if you're still unsure which nib is best for you.