I don't think you'll hit a "size limit" since the default
menu-background.jpg is 3000x1536:
https://osu.ppy.sh/wiki/File:Menu-background.jpgpause-overlay.png and
fail-background.png: The size I'm using for the standard images are 1920x1080. You could add @2x versions and see if that makes it work.
menu-background.jpg is supporter only but the same applies... You could add a @2x version and see if that makes it work. My standard menu-background.jpg size is 2880x1634 and it's worked for years.
For the Taiko Drums:
https://osu.ppy.sh/wiki/Skinning_Taikotaiko-drum-inner.png:Default size is 90x200, Centered Horizontally, Placed at the Far Right Border of the image, and Facing Outward
taiko-drum-outer.png:Default size is 90x200, Centered Horizontally, Placed at the Far Left Border of the image, and Facing Outward
You can look at the default images for taiko-drum-inner.png and taiko-drum-outer.png in the Taiko Skinning link above to get a better understanding of what I mean.
