how to calculate the scale size of sprite as per above info so that sprite look good

If your sprite is rendered by a Sprite Renderer component, and you know the pixel dimensions at which it "looks good", there's a nice trick you can use to "calculate the scale size" corresponding to those dimensions.

Change the Draw Mode to Sliced in the inspector. Then change the Scale to x: 1, y: 1, z: 1. Then input the size that you like under "Size".

Now when you change the Draw Mode to Simple, the Scale will automatically become the way you want it.