Resolution for UI sprites?

When creating a new image in Photoshop/Illustrator, specifically for HUD/UI what resolution is best for higher quality. So for an ability thats going to always be on bottom of screen until player uses it. I was thinking just 64p x 64p. But i was wondering if that would look jagged and low quality. What do you guys recommend/use?

 

You should not think about the size of actual sprites. You should think about the actual resolution a player can have.
Choose a resolution and use it as reference. I always went for Full HD. But nowadays you probably want to optimize for 4k (if not for mobile).

So, create an image in Photoshop with your reference resolution, maybe put a screenshot of your game inside or of a game which is similar to yours.
Then create UI elements directly in that big file. Now you have a better understanding how big the elements should be. and you can simply extract them from the big image.

Note: if you use Sprite Sheets you do not even have to have the sprites in power of 2.