What size of image should be for one HD background image for an 2D game?
Here is the screenshot should i lower the res a bit.
Gotta be honest, I probably wouldn’t take that approach to creating a background. You can get a nice gradient by using a really wide line renderer. The clouds could be planes with a cloud texture on them, then you could move them around as needed and achieve a parallax scrolling effect by layering them and moving them at different speeds. 
OK thanks
actually i got an idea I could just add on them smooth follow so i only have blue background and clouds following player?
(trying to help you out) THAT LOOKS REALLY UGLY DON’T USE THAT lol anyways use layers and smooth follow etc and you can get a really nice effect! :)!
Yeah i know it was only qucik test i added better ground textures and other.