Custom Social Icons specifications

Hi, I'd like to have a Yelp social button, and since there isn't one provided, I'm trying to create a custom social icon. But if I use a large-ish one so that it looks clean, it displays it at full size. If I shrink it, it displays it the same size as the provided icons but the top of the box is cut off, and it just looks icky because the resolution is so low. Please let me know the specifications I should use for my custom icon so that it looks as nice as the provided ones. Or add yelp to your provided ones. :D Thanks! p.s. the social icons are in the footer widget: https://miboxdallas.com