In character customization, you can change certain features about your character. His face, eye colors, hair, muscular build...

So wait, what if I make my character an absolute monster, with all sliders to make my character look as strong as the game allows. At level one, will he be as "strong" as a bosmer with all sliders set to make him look like a twig?

Do you think there will be any number crunching beneath this slider or is it purely aesthetic?

aesthetic

I'd assume it'd be purely aesthetic.

I wouldn't mind seeing these sliders being a more behind the scenes deal, like in Fable (though not to the extreme Fable went to) you became strong or magical that reflected back on your character.

I expect it'll be purely aesthetic. If you want your 'pure mage' to look like Conan they can. If you mould your appearance to fit your character you can.