Download and share many Teeworlds grid templates. Create your own teeworlds assets with these templates. Templates show you the positioning and borders, which you have to consider so that they are compatible with teeworlds
Also about the texture being double the size, it helps visually. I made screenshots on how worse the texture looks compared to a 4k tee skin or 4k game skin(or 2k, at this point it does not make difference)
Right. Obviously the minimum size ddnet recommends is 1024x1024 as well as you, an official designer. I recommend 2048x2048 (2k) cuz of my reasons and experience. Let's set at that.