You might be able to come up with figures for approximate geographic size by comparing routes taken by the time it took and the probable speed at which they're traveling.

PoR-wise I remember the sea route from Crimea to Begnion being pretty good for this purpose:

Having narrowly escaped Daein's grasp the Greil Mercenaries are pleased to feel the ocean beneath their feet as they set sail. According to Captain Nasir, the voyage to Begnion will take roughly two months.

So you could like, run a piece of string across the sea route they take, figure some average sailing speeds, take into account navigating the islands and reefs, and now you've got a "scale" for the Tellius map. :D