A nib might sound like the name for a cookie, but it is actually the part of the pen that is used to deliver ink to the paper and is made from metal, steel, gold or even platinum for high-end pens, such as the ones created by Montblanc. Most Montblanc pens have a nib--which is a tip, as opposed to a ball--but figuring out how to tell the size of the nib is a bit challenging unless you have the original box.
Understand symbols for sizing of nibs. The three main pen sizes are F, M or B for fine, medium, and broad, respectively.
Look on the cardboard label on the outer packaging. You will see a small sticker on the barrel of the pen. This works only if you purchased the pen new or have the original packaging.
Study nib sizes by looking at pictures. Use the Internet, books, catalogues and other reference sources to get a sense for sizes. See Resources for two online Montblanc catalogues.
Look at the nib tipping material size. A wider or thicker line denotes a larger nib. Visit the Montblanc website for buying new nibs. It has an interactive tool which allows you to see how the different sizes of nibs affect your writing.