There is a fairly easy solution to your issue actually. TextBox item (along with HtmlTextBox and CheckBox) have CanGrow
property. Setting this one to false will prevent the item to grow and will just cut the text in your case. For more information about shrinking and growing items you can check this frequently asked question
As a best practice we can recommend to play a little more with the designer and after that to check the resulting code in the designer.cs (or designer.vb) file - that way you will get to know the product much better and you will be more comfortable with the options that each item provides.
the Telerik team
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