Hi Andrea Ferrari,
Thank you for reporting this issue.
In the CTP version RadRichTextBox can not be added to Horizontal/Vertical stack panel or in Grid cell that has Auto size, which is most probably the problem in your case. In the next version we will make sure this is not an issue. However, you should be careful when using this kind of panel with RadRichTextBox, since this way you will loose the UI visualization support, which may lead to poor performance and large memory footprint of your application when editing larger documents.
We suggest you use the grid panel and specify exact, *, or maximum size of the cells to avoid such problems.
Let us know if we can help any further with this issue.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.