I’ll explain why Derpibooru’s Image Upload tool didn’t fetch the actual size of an image from DeviantArt properly…

For somehow reasons, DeviantArt has changed/updated the ways of the Download button behavior (dunno which time that they’re updating this behavior, since I’m not working with dA at all) and for now on, when you press the Download button, it no longer redirects to the full actual size of the pic, instead, it will actually give you a Windows pop-up for which directory where the pic can be saved in the computer, which kinda annoying. Therefore, Derpibooru cannot fetch the actual size of a pic from DevianArt’s Download button, instead, it fetches the preview size of the pic from the pic’s main dA page, the only way to solve this is to download the pic first by clicking the Download button on DeviantArt and save it on the computer, if the pic doesn’t have a Download button, then you need to zoom in the preview image and see if it got a bigger res hidden there, if the pic has that, right click and save the image to your computer, then go to Derpibooru’s Image Upload tool, copy-paste and fetch the dA link of the targeted pic, finally, when the pic from dA shows up here along with the artist tag, upload the downloaded full-size pic that have been saved from the computer by clicking the “Browse…” box, this should help to fix this issue.