The source on fanbox has a 130MB JPEG file which would've been impossible to upload.
However, the artist also shared the PSD file which allowed me to export a JPEG at quality 96 to make it fit within the filesize limit.

Ideally I would've used the PSD to create a perfect PNG, but that would result in a 160MB file

11024x6299!? Damn, the guy (the artist) really likes Anna, huh?

Would this be the pic with the largest file size ever to be uploaded to Danbooru?

SerialBus500 said:

Would be the pic with the largest file size ever to be uploaded to Danbooru?

It's currently in place 24 when it comes to filesize.
The largest at this moment would be post #510341 with 101 MB which was posted a long time ago before Danbooru had a filesize limit.

And while we're at it:
The widest image is post #629027 with a resolution of 30000x466
The tallest image is a tie between ⚠ post #7163773 and ⚠ post #7163780 which are both 1000x40000

⚠: explicit bara