Life's a journey, not a destination. PC NA, PC EU (non steam)- three accounts, many alts....
Could test by walking across the zones from coast to coast. See how long it takes. Average the walk of a human IRL then see how far a human could walk given the same time.
I'd say it's the size of a small city in america.
"Objects in an MMORPG may be smaller/larger than they appear."
MMORPGs tend to scale some aspects larger than one would expect IRL to account for things like third-person camera. It's uncommon, though not unheard of, to aim for a 1:1 scale on general measurements -- I seem to recall Mortal Online II giving accuracy some kind of try (but not sure how true their accuracy is).
For measurements studies, ESO provides the luxury of a home system with measurements as small as 1 cm, and I've used this to figure out the height of a max-slider orc... which is 240 cm or around 94.5". I don't have anything lore-based to reference, but I don't recall Orsimer being up to 7' 10.5" tall -- and that's not including examples like Abzag the Monster who stands taller than the sliders a player can access (or what I call "Undaunted Height").
In short, it's a game, scale is weird in games unless the developers make a pointed effort for 1:1 accuracy with Earth measurements, so Tamriel and Nirn are "as big as the developers feel like making it."
Why should Nirn be compared to earth ?
And if nirn is flat, how about borders ?
we know there's a celestial vault and that nirn is a plan, not a planet.
I think the question of the size doesnt really matter