Can you explain what you mean by "length of my Keynote slides?"
If the slide show plays automatically, then the time displayed is whatever you have set or recorded. The slideshow counter on a two display setup will tell you what that time delay is, but I don't know if you can simply scan the slide and see that info.
I seldom use anything other than manual advance, so the timing is not relevant.
If you mean physical size of the slide, then that doesn't quite make sense in this context.