Hi Graig,
When you're creating the design in CSS Sculptor, you wold just choose 'fixed normal' instead of 'fixed full width'.
But to adjust an existing one, you would need to add a width property to the footer div, and add auto right and left margins.
Supposing your outer wrapper has a width of 88em, then you would adjust the #fixedFullWidthFooter css to read:
#fixedFullWidthFooter {
margin-top: -17px;
position: relative;
width: 88em;
margin-right: auto;
margin-left: auto;
}