The approximate line widths produced by these nibs are as follows: - Broad 0.85 – 1.00mm - Medium 0.65 – 0.80mm - Fine 0.45 – 0.60mm - Extra Fine 0.35 – 0.40mm These are approximations only and will be influenced by the amount of pressure exerted by the writer and the type of ink and paper being used.
Please note:  the thickness of the line depends on the pressure exerted on the pen, and the type of paper so the above is guidance only.
As a rule, a Broad nib suits large hand writing and flowing signatures, Fine nibs suit smaller hand writing and figure work. Broad nibs can tend to fill in the spaces in “b”, “d”, “e” if you hand writing is small. Medium nib is a good compromise, and selected by the majority of fountain pen users. Please see here for a diagram