One of the old classics in this genre of largest factories, BASF, https://www.basf.com/jp/en/who-we-are/organization/locations...
Volkswagen's main factory is also pretty large: "Spanning more than 6.5 km², the Volkswagen factory in Wolfsburg is now the largest automotive plant in Europe, employing more than 60,000 people."[0]
[0]: https://www.volkswagen.de/de/marke-und-erlebnis/volkswagen-e...
It becomes tricky because what does 'factory' mean? Especially once it's scaled to the size of a small town, you start to see town-like buildings popping up. There's some stuff which is arguably necessary for the factory itself, such as the train station and logistics facilities, a test track, and things which are necessary for the employees directly such as employee car parks and restaurants, but then there are other things like a visitor centres, shops, gardens, monuments, museums, a Ritz-Carlton hotel. Also, are power and water facilities part of the factory? Even if that means that we're counting a reservoir as 'part of the factory'[0]?
How would you come up with an exhaustive list of which buildings contribute to the size of the factory, and which should be excluded?
Otherwise, you're just looking at the land owned or leased by the company, which is obviously a valid measure somehow, but it's hard to say whether it would lead to fair comparisons. I don't have the answers, just throwing it out for debate.
Of course, it becomes tricky to decide what belongs to a factory or not if you look closely enough. And comparing factory sizes is something inherently silly. But I think that in a capitalistic society the land area owned by the company is a good first order approximation. If they wouldn't need it, it would be better for them to sell it.
But in the case of the Volkswagen factory the points you mention lead to something interesting: The city of Wolfsburg was created just before WW2 to house the workers of the factory created at the same time to produce the precursor of the VW beetle. Even today the city is dominated by the factory: it has a population of 120,000 and 60,000 work in the factory. So under a loose enough definition of "factory", the whole city of Wolfsburg is part of it.
Which if this BYD site is 2x2 miles for ~5sq km, BASF is twice as large at 10sq km! Wow