What do they mean exactly by Fret Size, like Jumbo, Medium... Is that how wide they are? Or how big the fret/piece of metal is that defines the frets?

Also, when they refer to a scale size of a neck, such as 25.5, is that how long it is? So for example if a neck has a 25.5 scale but 22 frets, are the frets wider than a 25.5 with 24 frets?
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There are many types of fret wire and the dimensions used when discussing frets are the height of the crown, the width of the crown, and the general profile type of the wire. To say medium, large and small is a generalization of fretwire. When building guitars, you usually know the exact dimensions of the wire. Different fret wire manufacturers have their own monikers for each wire they use, but it's not uniform among all companies.

The scale length of a guitar is known by measuring the distance from the point at which the string breaks over the bridge to the nut of the guitar. Each individual string, when intonated properly, will have slight micro differences. The logrithmic formula used to determine fret placement is determined by this distance. Number of frets have no bearing on scale.