It appears that you can't - slide dimensions are always set in pts.
However, what is it you're trying to do, specifically?
It doesn't ordinarily make sense to size a slide in absolute terms - either a specific number of cm, inches, etc.
Everything on a slide will be sized proportionally based on the output/display medium.
For example, you may have a slide that's conceptually 30cm x 20cm, onto which you place a 5cm x 5cm object in the center. - when you present that slide, it's going to expand to the size of the display, and all objects will expand proportionally - so if your display is twice the size (60 x 40 cm) then your 5cm box will now be 10cm to maintain the same proportion. Similarly, if you print it, the slide may get scaled down to fit the page, and the object will be scaled down, too.
The same goes for text - even if you set a text object at, say 72 point on a 1920 x 1080 pt slide, the actual display size will depend on the output media, and the text will retain its 1/15th relationship with the slide (1080 / 72)
So as far as Keynote is concerned, all measurements are relative, so it's not clear what you expect to gain by specifying a slide as a set number of cm.