Keynote control text box size

Short answer, use a rectangular shape as you were to draw a block, and then just type words inside it. Then you can change its size freely. However, this is NOT called textbox in Keynote.

The confusion/frustration comes from how Keynote and Powerpoint handle textbox. In Keynote, I think a textbox can't be changed height wise, since it's supposed to big enough to expose all the texts in it. So you can't arbitrarily change its size. In contrary, Powerpoint let you change the size of a textbox freely even if you block some of the texts in it.

Mac OSX Yosemite 10.10.3

Keynote version 6.5.3(2151) (don't think these matters in this case)

Selecting the box turns out to be easy (but is undocumented). You just click outside the box first, and then click inside the box. If one then selects text inside the box, one is able to select a tab labelled Text, when you can control Font and Font Size. Other tabs control other aspects of the text.