What do you think about phone number input field’s width in web design? Is it important whether the number is long or not?

I'm designing a new form field for sign up page and something disturbs me. I have to show phone number field but I don't want to make it wider than phone number's length. I realize most web sites don't take care of the width and they try to fit other inputs (such as name, email etc.)

What do you think about it?