Ok. So for the default one, you will need access to the sliced sprite, font, etc. I suggest making a behaviour and attaching it to the root object if you want to do this, to make it easier and reduce guesswork.
About dimensions, that is easy enough to change on the sliced sprite. After you change dimensions, you'll need to change collision box size as well - you can do that the way it does it in the editor, or calculate it yourself using the sliced sprite dimensions and CurrentSprite.TexelSize - this will be considerably faster than calculating by bounds.