Q: iPad photos sort by size, largest disk size to smallest?

Spare yourself the frustration; use 3rd party apps like Photo Manager Pro. You can arrange the photos like you arrange your apps.

https://itunes.apple.com/sg/app/photo-manager-pro/id393858562?mt=8

I believe iPad photos are sorted by the modification date and this cannot be changed. If the order of photos is important to you I suggest an alternate photo app. I use photo Manager Pro.

If you want to sort your photos by size, first create a new album. Move your photos into that album. In the new album tap select, hold your finger on the photos to move them, tap done. Whatever you do don't erase the photos from camera roll.

Here's what I do:  I use Image Capture to copy all my iPad photos to the Mac.  Then in the Finder I sort by size and then I know what files (& most importantly the dates they were captured on, since that's how iPad sorts photos) and then I can delete them from the iPad (if I already have them on my Mac) in order to free up space on the iPad. 
This is my best recommendation using only Apple products.  

It would be nice if Apple provided this functionality on the iOS so we don't have to do it on the Mac, or rely on 3rd party apps to do basic file management.  And you would think that Apple understands the importance of sorting by size since that's how "Manage Storage" works on iOS, sorting by size (largest at the top).  Just give us that same functionality in the Photos app.