In your token-making program of choice, just keep the canvas area the same size but make the image itself half as big. For example, I build my tokens to 280px by 280px (so they retain good, zoomable detail but still look nice at the default 70px size). For most tokens, that means that there's just a little bit of transparency at the corners, with the token itself being 280px in diameter. For small tokens, it's still a 280px graphic, but the actual token is 140px in diameter amid a sea of transparent pixels. I don't have the MM addon, but I'd imagine that's how they're accomplishing the stirge's snap-to-center-ness.