Question: I want to make text smaller, not zoom diagram but change font size.
Is it possible?

Answer: 
I've found a solution:

Open \*.cd file with text editor.
Find string in the end of file like this:

```
<Font Name="Consolas" Size="7" />

```

Change it.
You can change this by setting font size and more in the options menu of VS:

<http://social.msdn.microsoft.com/Forums/vstudio/en-US/566e3e74-5955-431f-bc58-f92f68effa39/changing-font-size-in-class-diagram-vs2012>

Reopen your diagram and everything will be adjusted.