Which resolution to choose for sprite assets creation (mobile)

hi,

I wonder which resolution to choose for the creation of my sprite assets (e.g. in photoshop)?

And what is the actual workflow: I create a sprite in a super high resolution (to support ipads, retina,...) but export it to unity in lower resolution...? or how does it work, since if I import a 3072x3072 sprite it will take 10mb memory or more. Wouldnt I run into problems on mobile?

Also what is the advantage/disadvantage if I use the Pixels Per Unit value compared to the transform.scale on the sprite, both will modify the size?

Can you recommend some tutorials to this topic or the general workflow regarding photoshop->unity

Thanks for your answer.