Of frets...

The metal fittings in our guitar fingerboard, are the frets. The typical cross-section of a fret is pictured above. If you view your guitar neck from its sides, you'd see the above profile, less the barbs, which are hidden by the board's wood. The function of the barbs is to hold the fret in place after it's pressed/ hammered in. This leaves the bead/ crown exposed to come into contact with the strings. 

Before the frets reside in our guitar necks, they are cut from a very long piece of metal also referred to as the fret wire.

However, it's more convenient (not to mention economical) to sell fret wires in an already cut-up form as practiced by Dunlop, the leading fret wire manufacturer today.

When we talk about fret sizes, we are referring to the measurements pertaining to the crown's height & width. The exact size references are numerically assigned as depicted above. The layperson references are simpler; 'vintage' being the most restricted & 'jumbo' the most generous of them all. The contemporary electric guitar sports a fusion of both ends, the 6105 size is rather popular as it allows a good grip & not being overwhelming in feel, Joe Satriani has this in his Ibanez JS models. 

There are endless debates in countless forums lobbying for the 'best' fret size but it's all a matter of preference, much attributable to the individual's playing style. On a personal basis, I am adaptable to all fret sizes but inclining towards the jumbo & its derivatives- these supplement fast playing styles especially so for us dweebs with light touches.