That's entirely personal preference. It's a fact that an 8.9" is more portable than even a 12.1" but as far as being usable, that's up to each individual user.
I personally would not go below 14", and my 15" is quite heavy as it is - I'll bring it around but I really don't feel like carrying it EVERYWHERE with me.
However, my friend has a 12.1" and wishes to go even smaller. I pointed out the screens and keyboards would be incredibly tiny - he saw one for himself and said it would not interfere with his ability to work, and in fact, would enhance it due to its greater mobility.
The OP has said the 8.9" was "perfectly sized" so he should try to find as much information about it as possible.
To the OP:
Here's a preliminary result from NotebookReview.com, I assume this is the one you're looking at
http://www.notebookreview.com/...HP+2133+Mini%2DNote+PC
It received quite a nice bit of praise on its build, and also has benchmark numbers there for you. Unfortunately, the numbers don't look quite as bright. 3D performance via the 3DMark benchmark is downright awful.
To those concerned about the small keyboard, here's an excerpt from the relevant sections:
[...] the keyboard is almost full size. It is 92% of a full sized keyboard, which is quite impressive for such a small form factor. It is much more comfortable to type on compared to the Asus Eee PC. [...] The keyboard on the 2133 Mini-Note is great. To put it plainly, this is the best keyboard we've seen on a notebook this small. [...]
EDIT:
I am aware that the keyboard is also another thing that is HIGHLY personal, so take that last paragraph with a grain of salt, but at least it wasn't reviewed off the bat as incredibly terrible.