Line up the multiline edit box's top and bottom edges with the list part, and give its left edge an offset of 10 from the right edge of the list. To use all the remaining width of the window part, attach the box's right edge to the parent's right edge, using an offset of 10:

If you test the window, you will see that the visual parts of the window grow and shrink with the window. If the font or font size changes or text changes due to translation, you would also see that the parts resize and position themselves correctly.