In most cases, manufacturers’ stated diameter for monos/fluoros varied marginally, if at all, from what my electronic calipers measured. And, somewhat surprisingly, where discrepant, the measured sample often proved smaller than the spool’s stated diameter.
For braided lines also, the difference between stated and measured diameter differed less than I would have expected, especially considering that braids can be trickier to measure. In a few cases, diameter measured much less than stated.
In many respects, the thinner a line for a given strength, the better. Smaller lines allow more capacity on the spool, are likely to cast farther and easier, and offer less water resistance. On the other hand, thinner lines generally offer less abrasion resistance, since any scrape or nick takes out a larger proportional chunk of them.
Also, this chart indicates whether each line is a monofilament or a fluorocarbon, and whether it’s sold as a main line or (in spools with less line) as leader.