keynote control text box size

How do I control the text box size in Keynote. My main problem is that I can't even select the box, though I can select text within the box. When I click on the paintbrush on the upper right of the keynote page, and then select Arrange, the dimensions of the textbox are shown, but greyed out.

Very frustrating. Of course, Keynote Help is no help. Usually Google can find the answer in such situations, but not in this one. There is just an assumption that you know how to select the textbox itself, rather than the text inside the box.

I am using OSX 10.9.5 on a Macbook Pro. My Keynote is version 6.2.2. I do get messages that I should update Keynote, though I would first have to update my operating system. I hesitate to face the inevitable disruption just before giving a big presentation before a large audience, perhaps an unwise decision to try Keynote for the first time on this occasion.

Thanks for any help.