Touhou Headcanons!

What's the biggest mansion?

Gensokyo has a lot of mansions, eh? Well this isn't very relevant but hey, what makes a headcanon, right?
Right?
No?
Okay, fine.
I'll just roll with it.

Here's a map to guide each mansions' location (in exception to Chireiden probably)

Scarlet Devil Mansion (Koumakan)
Personally, this can be counted as a big mansion. It takes a european style along with combinations of scarlet, red, and crimson. They also have an invisible chimney, probably at the back of the mansion.
The noteworthy is that it's near Misty Lake, which makes Koumakan not very visible from afar. It's also near Youkai Mountain and Youkai Forest.
If we look back into the map, comparing the size of Koumakan is no comparison to Eientei.

(An absolute lense flare indeed. Yeesh....)
House of Eternity (Eientei)
Closer observation, Eientei seemed a lot bigger than Koumakan. But in truth, it's just wider. Adding Kaguya's ability would make Eientei the ‘biggest’ mansion. Although magic affecting the building itself isn't my top criteria.

Anyways, it takes a traditional Japanese building, like Himeji castle or something. Well, it isn't technically similar.
The rooftop's colour is brown, supposedly. But if it's blue, then maybe it's canon? But if we see the inside,

It looks too brown for a rooftop to be blue coloured.
(yes it is from AoCF, because it's the easiest access.)
So Eientei's size comparison to Koumakan? Koumakan's more big, but there's an even bigger one.

Palace of Earth Spirits (Chireiden)
My favourite mansion!!
Down the geyser (in the map, near the Hakurei Shrine and Bamboo Forest of the Lost), go through Yamame's web, Parsee's bridge, and the Former Capital, here we enter Chireiden.

(Yes I took it from a minecraft video, so?)
Because there's no official map for the underground, as a result, this is what I took.
Chireiden is, without doubt, the biggest mansion in Gensokyo. Why? It is connected to the Former Hell, the Geyser Center, the Core, and the Geyser launch altogether. The entrance is through Parsee's bridge (which is absolutely beautiful) and through Former Capital. And Former Capital is like a city to Chireiden.
And if we add all of them, it is clearly, without doubt, the largest mansion in Gensokyo, despite its location.

Halls of Dreams' Great Mausoleum (Shinreibyou)
Connected to Halls of Divine Spirits, Miko's Dojo for Taoist members. The taoist gangs' actual house is in Shinreibyou.

It is also connected to the Netherworld, which explains why the access is through the Netherworld during Ten Desires. Indirectly, it is connected to the underground altogether. But Halugyokurou doesn't belong to Miko, so it's clear that this mansion is the tallest. If we're to include Miko's Dojo, it'll definitely increase the width but it doesn't change much as Eientei nor Koumakan.
But hey, at least it's the HIGHEST mansion, amirite?
So high, it reached the clouds, which is why I said it is connected to Miko's Dojo.

Hakugyokurou Manor (Yuyuko's Manor)
The exact size isn't confirmed but you can definitely see it in the map. Anyways, this is Yuyuko's manor!

If she owns the entire Hakugyokurou, it'll overpower Chireiden's position. Thus being the largest mansion in Gensokyo. But once a manor, forever a manor. Manor is a smaller version of mansion, thus excluding Hakugyokurou in entirety, Yuyuko's Manor is awfully small. But big enough for comparison.
Do you think Hakugyokurou belongs to Yuyuko?

Myouren Temple (Myourenji)
I could simply include Palanquin ship altogether. Anyways, Myourenji!
Palanquin ship isn't exactly a mansion, more of a ship controlling the flights and movements for Myourenji, since Myourenji is located exactly on top of it.
(more like Myourenji being on the ground but able to be moved to Palanquin Ship.)
And if you look at the BG image of it, you'll notice how huge it is. Shamefully it doesn't overpower Chireiden's position.

There are actually other mansions but I'm picking major ones because they're more noticeable than the minor mansions.
Conclusion : Chireiden's the biggest mansion. On top of it, Satori cares a lot about the Former Capital, along with Parsee's bridge. So it pretty much adds up how she technically ‘owns’ Former Capital and Parsee's Bridge. Unless someone claimed Hakugyokurou belongs to Yuyuko, Chireiden will remain in the lead.
I don't have any proper screenshots for Yuyuko's Manor, so for now, use the map I placed on the very beginning for some interpretation.