Question: I read about these several questions, how to use a font size other than 10, 11 oder 12 (see e.g. [this one](https://tex.stackexchange.com/questions/5339/how-to-specify-font-size-less-than-10pt-or-more-than-12pt)) using KomaScript, but somehow, that won't work for me. I use XeLaTeX to compile my document (in order to change the font also, but the problem already occurs without that).

Then using the MWE

```
\documentclass[fontsize=13,DIV=12]{scrbook}
\usepackage[no-math]{fontspec}
\usepackage{xunicode,xltxtra} 

\begin{document}
    Dieser Text sollte in Schrift 13pt sein, ist aber 12pt
\end{document}

```

The warning then is

> 
> LaTeX Font Warning: Font shape `OT1/cmr/m/n' in size <13> not available
> 
> 
> (Font) size <12> substituted on input line 1366.
> 
> 
> 

Which i suspect is due to `OT1` but how do I tell XeLaTeX to use `T1` (also because it's a german text, as the MWE might already suggest)?

Answer: 
You can ignore the message: It is only a side effect of the calculation. The font itself is in 13pt:

Add 

```
 \fontsize{12pt}{12pt}\selectfont Dieser Text sollte in Schrift 13pt sein, ist aber 12pt

```

after your text to see the difference. 

If you want to avoid the warning altogether add `\RequirePackage{fix-cm}` before `\documentclass`.
To get rid of this message you can write an own file `scrsize13pt.clo`. As Markus Kohm said in his printed book (German version chapter 17.1, page 428; sorry, I have only the book in German) KOMA-Script with the option `fontsize=13` or `fontsize=13pt` searched for a file `scrsizexxpt.clo` or `scrsizexx.clo` with `xx` stands for the given size, here `13`. If it finds this file a recalculation by KOMA-Script is not nececcary and you will get no warning.

You can use file `scrsize12pt.clo`, copy it to `scrsize13pt.clo` and change the values to 13 pt. Please be careful this it not so easy: You have to define for example for all fontsize macros like `\normalsize` or `\Large` the propper values.