By the measurements, it appears to be slightly smaller than a GBA SP, plus the extra width, of course.
I'm very glad that they moved the D-pad and buttons up higher, because their placement on the DS made it absurdly top-heavy. The reduced weight is also a big deal.
I personally think they still could've done more to make it comfortable (this probably still has the pathetic little D-pad and buttons Nintendo seems to have gotten used to), but it looks like a very good balance of comfort and appearance (the old DS had neither, IMO). And the fact that it looks like a GBA SP is a great move.
The MSRP seems a little high, and NOA in particular should be looking for a price drop, because while this redesign is very nice, I don't see it holding Sony at bay for very long, especially not if Sony has any surprises of their own in the works.
Edit: I wonder how the L&R triggers are? Hopefully they don't "pull an SP" in that regard. It seems unlikely. And where's the power button? Don't tell me they "pulled a Micro" and put it on the lower edge of the unit? And while I'm complaining, I hope they ditched the "slider" volume control and moved back to a dial.