HDRI Backdrop issue

From what i have seen, the original texture/image/cubemap resolution (before importing it into the engine) becomes the max ingame resolution, unless the imported texture is way bigger, than what the engine can handle, then it will be the max res the engine allows (was it 16k?).
Don´t ask me why it always uses a squared resolution for cubemaps, which obviously are not squared ^.^ I guess, thats an engine thing, how it handles all texture (identical dimensions for all sides), and the engine makes sure, that you don´t create a texture, that is bigger/higher than your original image, that you have imported.

Or if it is in between two resolutions the engine supports, then you can only go with the next lower one. Once i had a HDRI with 10000x5000, and the engine did only allow 4096 as max resolution. The next higher res would have been 8k, which would have been way higher than the original height of 5000 pixels from the HDRI, so i could only pick 4096 in that case. Ok, that was UE 4.19 or so, so not sure, if this still is the case, but i think so.