I believe I might just be able to work out the size of High Lagaard. You know EOIV, right? In it, you can travel in an airship all around the place. Sometimes, you can encounter one of the three dragons. Each one of these is massive; taking up a 3x3 space, despite the labyrinths and the town of Tharsis only taking up a 1x1. So the dragons are 3 times as long as Tharsis. I'll look at how big Tharsis is later.
These same dragons can be found inside High Lagaard's labyrinth, which is set inside Yggdrasil. They also take up a 3x3 space in that. Since the whole labyrinth is made up of 35x35 space maps, its safe to say that High Lagaard's Yggdrasil is at least 35 times as wide as Tharsis. And that's just the Yggdrasil; High Lagaard itself is about 9 or 10 times wider than the Yggdrasil it suurounds. Take a look at
this image and you'll see. Let's go with the low end, 9. This means that High Lagaard is at least 315 times as wide as Tharsis, or 99225 times bigger than it overall.
So how big is Tharsis?
This picture gives a fairly accurate representation of how big it is. Assuming that one of those trees is about 10m high, I'd estimate the town to be about 180m wide, or 32400m^2. That seems like a fairly average size for a small town.
Feeding those values into a calculator, it seems like High Lagaard is about 3214890000m^2, or 3214.89km^2. That's about 1.5 times as big as Tokyo, meaning it's larger than any real world city, but not outside the realm of possibility. I'd say that that's large city level.