The recommended height is 200 pixels. What about the recommended width?
The standard used to be 3000 pixels. These days I use 5200 pixels if I’m going to have a pattern or texture across the whole image.
However, the rules kind of go out the window if you use Firefox Color. The recommended height is still 200 pixels but the width can change greatly depending on what you are trying to create. I can help if you have specific questions, but I also have a few tutorials online that might be helpful.