Hello Heiko,
This problem appears when RadEditor is set to use the
same
default font and size, as the font and size that are set in the
content editor's browser. Then the browser doesn't use a
font name / size tag to display nor save the correct font as it recognizes it be
default. The problem occurs when another user with different browser's FONT
settings accesses the page. Since the new browser thinks that the default FONT
should be, say Times New Roman, it displays it with the new settings.
I can suggest you the following work around. You need to change the default font-size in the RadEditor. In order to do so create an CssEditor.css file in the Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\4.5.6.0__1f131a624888eeed\RadControls\Editor folder and set the following css style.
For example (you can change the size so it will fit your case):
I hope this helps.
Best wishes,
Stanimir
the Telerik team