14mm is considered a SMALL size crystal.
16mm is considered a MEDIUM size crystal when in chains.  It is neither small or large. Perfect for most projects. If you are unsure what to pick, this is usually the best choice.
20mm is considered a LARGE size crystal when in chains
24mm is considered an XLARGE size crystal when in chains.
26mm is considered an XXLARGE size crystal when in chains.

Crystals are sold in mm sizes.  MM size refers to the length of the crystal in most cases, depending on the shape of the crystal. 

For octagon shape or round shapes, the mm size given is both the width and length of the crystal.  For Teardrop shaped crystals or Almond shapes which are longer than they are wide, the mm size is referring to the longest dimension of the crystal from top to bottom.  The pink chart shown, approximates what some common crystal millimeter sizes are similar to.

To see in front of you, what each mm size looks like, you can get a coin from your wallet or purse.  You can also look at each product description and you will see the mm size broken down into fractions of an inch.  For example, 14mm products also state that they are .55" inches.  This means that 16mm is the same as .55" on a ruler.  Look at a ruler at the 1/2" inch mark and note that 14mm is .05" of an inch larger than 1/2" an inch wide. 

How Does MM Size Affect the Finished Result & Sparkle?

Assuming the crystal grade is the same, meaning all the same exact brand and quality, the larger the mm size, the more sparkle.  Meaning, if comparing 14mm Swarovski to 16mm, the 16mm seems to have more of that dynamic sparkle.  When comparing 16mm Swarovski to 24mm Swarovski, the 24mm would have much more sparkle and so on.   If the cut pattern is the same on both and if both have the same number of facets, the larger crystal will have larger facets.  Being the same quality and brand and having both lots of facets and larger facets means more sparkle than the same number of facets but smaller mm size of the same crystal.

Does This Mean Bigger is ALWAYS Better?

No.  This is only true if you are comparing apples to apples where quality is concerned.  You can only compare the same brand to the same brand, just differant mm sizes for this to be true.  An inexpensive crystal of a larger mm size will not have more sparkle than a higher quality crystal that is a smaller mm size.  Superior quality always trumps mm size.