I agree with those who say get a 21 for your first CRK. The pivot bushing alone is enough reason. It is truly a great piece of engineering. That is not to say the 25 isn't, they are just two different animals. The large is noticeably larger than a small, but not huge by knife standards, if that helps at all.
The large 21, the 25 and the Umnumzaan are all the same size as well. The only small currently in production is a 21 or a Mnandi (or the soon to be released Inkosi).
As far as the pivot/lockup goes, please look at nyefmaker's post in the Inkosi thread, fifth post down on page 2. He explains it well:
http://www.bladeforums.com/forums/showthread.php/1289669-Inkosi/page2
I would also vote for Micarta inlays if you are going to be using this knife. They make the knife even that much more great IMO. I could never get into the carbon fiber scale Sebenza's. I've owned a small, gave it a chance, and ended up trading it away. I guess I just prefer titanium.
I'd recommend buying one here on the exchange. You will get a discounted price and can always get most, if not all of your money back if you want to try a different CRK.
I am sure you are going to be happy with whatever CRK you choose.