This type of functionality is not supported out of the box. You have, however, a couple of options which could help you in achieving a similar result.
You could build a complex layout in a RadSplitContainer, dock your textbox is some of the panels and use the splitters to resize it. This article could help you in creating the layout: Building Advanced Layouts. Another option is to make the control automatically resize when it is full with text. Please check my code snippet below as well as the attached gif file: