Thank you for writing to us.
This is an known issue. It will be addressed in one of the next major releases. In the meantime, you can use Visual Style Builder to create your custom shape. Then you should create a custom shape at runtime with the defined shape dimensions. I have attached a recorded video that demonstrates how you can do it.
Let me know if you have further question.
the Telerik team
Check out the Q1 2011 Roadmap
for Telerik Controls for Windows Forms.