One of the main goals of the classicthesis template is to provide a typographically pleasing document. No matter how buggy the implementation is.
The manual begs the user not to change the margins.

Nvertheless, if you want to change the margins to specified values, the recommended way with KOMA-script is to use package geometry indeed. But after loading classicthesis.

As cfr notes in the comments, neither \footnotesize nor centering do take any argument.

One of the main goals of the classicthesis template is to provide a typographically pleasing document. No matter how buggy the implementation is.
The manual begs the user not to change the margins.

Nvertheless, if you want to change the margins to specified values, the recommended way with KOMA-script is to use package geometry indeed. But after loading classicthesis.

As cfr notes in the comments, neither \footnotesize nor centering do take any argument.