I can attest to LordSuragaha's statement about the old maps, fluorite specifically is definitely the largest map there was/is as it was the ONLY map where a character with wings was a BAD idea, since it'd take upwards of 4 to 5 minutes just to get to the temple of dreams portal (I should know, I used to play the old maps and recently snagged the old maps launcher over in xSpirit's post, fluorite is massive)
It's not really the size, but the quality of the map itself you should be concerned with, or, not be concerned with, as the new FH maps have considerably more detail and resources to make them feel more lively and like an actual environment instead of a giant empty open field with a few hills
And I also agree that larger maps would just mean a more widespread player base where it'd be a lot harder to find players to interact with, which my example is sky's rim. The map before it, ascension island, was a massive upwards parkour challenge that led up to sky's rim, and not many players had the determination or the drive to get to the top, so sky's rim was either empty, or had maybe a handful of players in it due to how hard some of the parkour jumps in ascension could be, which left dozens of players to flood the base instead
Honestly? With how tall some of the skybox invisible barriers are (Specifically in the grounds, it took like 15 or so seconds to fall all the way from the top to the ground) it would be possible to edit them in a way that would add more geometry and objects in to fill out some of that empty space and give you a larger map to explore while keeping the entire map's size (The square grid space it occupies within the game) the same