I'm using the latest LR on my iPad Pro, iOS 13.4.1
If i have use only smart previews turned on they are showing the full size dimensions, but when i export using the max size available the resulting JPG's are quite a lot smaller than the screen dimensions, roughly just under 2500 pixels long, and the files size is quite small.
If i turn off the Smart preview setting most of the time LR still only exports to the same size. If i type in a custom size, say 4000 pixels, the export works as expected (although never on multiple files).
Sometimes i have ot export the same files twice to force it to use the full size. This is happening on most, but not all my files. All are Canon .cr3 files stored in Adobe's cloud.