My view of large has changed over the years, since I started in the hobby with a 6' long 135g tank, I don't really feel the large thin rectangle that's 6 feet long instantly makes it a large tank, when the tank starts to get width or depth that exceeds the standard 18-24 inches is when I start to feel a tank is getting "large"
However this forum has made me question what is "large" for me large is a size that takes a lot of effort and care to do, and quite honestly seems like 6 feet is a standard size in "normal" aquariums, yes it's on the bigger side of "normal" but it's not obscenely large. When I started this hobby approximately 10 years ago my draw would have dropped at a 180g tank, just the thought of 180g not a TOTM version, even though I had a 135g that seemed so much larger. A few years later I saw that guy from Thailand set up his 1000 tank thread (Chingchai I believe, I apologize if I'm butchering the name, its early and I'm still tired), and that was literal jaw on the floor big, then there's the guy up in Canada in the 1000g+ crowd, and another in Arizona, and another in... and then there's an even group of those that are between 500-1000 gallons and all ideas of "large being 180+" kind of went out the window.
So large is a relatively term, while the above mentioned tanks are definitely in the "large" category, if you're in a tiny 1 bedroom apartment a 100g tank will be large. Many years ago all my fish stuff left the living area and went into the "laundry room" area that basically is a room that's 25' x 15' with no furniture or any of that other than washer/dryer/sink so the 80g, 100g, 120g, 180g, tanks I've had down there at various times never really seemed "large", the 375 system I'm setting up may feel large (i'll know it once I can see it completed) but even if it doesn't I don't think I'll feel the desire to go larger because it might not be large but it'll be big
That said, we have local tank tours around here and seeing some of the smaller tanks some people have puts me in awe of what they have. Size doesn't matter, if the tank looks nice who cares how large it is. You could have 30,000+ gallon outdoor tank, if all your rocks look brown and covered in diatoms and the water is about as clear as a glass of beer you got a big tank of algae soup. While the hardware side of things could very well be impressive, bigger isn't always better. The Steinhart aquarium near me has a tank that's upwards of 220,000 gallons, while that's impressive the smaller tanks interest me a lot more because they are more grown in and "look nicer", 5-10 years if they can get massive coral colonies growing and healthy then maybe I'll change my mind.
But to not be completely wishie washie on the actual question, I guess I would say if there's not a stock size in that dimension (ignoring a couple inches one way or another) and it needs to be custom made (again ignoring a couple inches), then it's a large tank. 6' x 2' x 2' 180g IMO, not really large, big yes but not large, 6'x3'x3' yeah that's large
You raise two points and I will address them separately.
