RadTextBoxElement) control uses internally the standard TextBox control (mainly due to its complexity). This means RadTextBoxElement has the same limitations as TextBox.
You can control whether the text box should appear on a single or multiple lines by settings the Multiline property (radTextBoxElement.TextBoxItem.Multiline)
If you need to "autosize" the text box according to the text, you shoud set Multiline=true and set TextBoxElement size, corresponding to the measured text rectangle. You can use the library method:
Let me know if this works for you.
All the best,
the Telerik team
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