How do you make perfect sized thumbnails / game icons

Through out my time in game development when making a game icon or phumbnail its always been too big or too small on width or hight and ive wondered how do you get the perfect sive for it.

this should go in #help-and-feedback:art-design-support

The perfect size is 512 x 512 pixels.


Ok thank you for saying about what the size should be.