When I use N810 with ukeyboard, only keyboard with the small letter keys shows up.
With my N810W (wimax edition) only finger friendly keyboard (with big keys) shows up.

Is it because of the different firmware (both flashed with the latest fw) or I just cannot find the right place to configure which keyboard I want to invoke?
How to tell the NIT I need small vkbd?

Go to Control panel -> Text input settings -> On-screen and check the "Enable stylus input methods". Optionally disable the "Launch finger keyboard with finger tap" option - it was a great idea but they never got it working reliably so it gives you the "wrong" keyboard half the time.

hmmm, it seems even if only "enable stylus input methods" checked, I get the big keyboard on N810W. I wonder if its wimax edition specific.

It's not, I have both and they behave identically. Did you disable "Launch finger keyboard with finger tap"?

yes, I did. Hmmm

There are two different virtual keyboards? I have only ever seen the small stylus friendly one!

There is a larger, thumb friendly one. Mostly used on the N800 where there is no physical keyboard (and center dpad is easier to access, as that is the button of choice for bringing it up without the finger tap setting).

How do I activate the larger one?

With a finger tap (if it is detected as such) or the center d-pad button (on N810* you can enable on-screen input methods with the slider keyboard open by pressing shift+space). The aforementioned launch finger keyboard option has to be enabled first. See Text input methods -> Virtual keyboards in the on-device help for more details.

On the finger tap thing, i have had most success on my N800 with already focused text areas and trying to get as much surface area of the thumb as possible on there.

And if one want the keyboard to always show up, there is a option for that as well.