Does Size Matter (in a Pen Tip)?
Well, maybe not that much to the graffitist in the next bathroom stall, but for us law abiding folks into particular pen pursuits, it does. Ballpoint and rollerball pens are generally available with tip sizes ranging from a "fine" or even "extra fine" to "broad." So, how do you choose? Here's some general guidance that is mostly common sense.
I choose a "fine" tip size pen when of course I want to write small or create a fine, detailed drawing of my mother-in-law and really capture every wrinkle. What may not be so obvious is the choice of a fine tip when you plan to use paper that is porous or readily absorbs ink. My daughter writes in Chinese characters as well as ancient text from Mesopotamia which is interestingly enough actually a Greek word, but don't get her started. With all the little strokes just so that distinguish one word or phrase from another, it takes a pretty fine pen and agile brain to get it right.
Logically, a "medium" tip is probably best when we're just relying on our good old alphabet-based language to craft our scintillating memoirs. This assumes your handwriting is "medium sized" (whatever that is) and you aren't including any detailed anatomical drawings for posterity.
As a general rule, tip for tip, a rollerball pen will produce a finer line than a ballpoint pen. For example, a medium tip rollerball pen will lay down a line about .7 mm wide. A medium tip ballpoint pen will lay down a line on the order of 1.0 mm wide.
I hope this fascinating primer on pen tip size helps to better your handwriting, drawing, or graffiti enterprise.