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)