Dont go large, the larger you go, the more roblox compresses and squeezes the quality out of it.
Instead, look for the size of the UI. You can see it by clicking on the UI which should (from the viewport) tell you in a yellowbackground, what the size is.
Example: 25 x 25.
Get that size, and use that as the template for the pictures you’re using. Then export it out at that size. Then you can use those images. If quality still isn’t enough.
You can try pixel fixing it to get rid of the black outline.
Tutorial w/ pictures if needed as well: