First, click on the text box to select it, you should see a red border with 8 red squares around the box. These squares are points where you can enlarge or reduce the size of the text box. You can resize the box by clicking on any of the little red resize bars scattered around its border and then dragging them.
You can also right-click on the text object and select the "Adjust Size/Position" option if you need to give the object a specific size or position.
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