I have yet to test my pony making but I can sure try. ^^
Dimension wise... They may be not be the exact you ask but I could try to make it as close as I can. Since I edit by adding clay onto or taking away clay from the base which would be my armature piece. Also it would depend, if you would like me to make her reading a book, in her gown, the positioning of her stance, and so on. *And since you added the "Princess" part I'm already assuming you would like the actual Ali-corn version of Twilight. So We'd have to discuss if you'd like the wings to be spread out and such.* I mean, there are many things I could do to make it any way you'd like. Well... except float. ^^; I don't trust myself accomplishing that just yet.
*My eldest is obsessed and as an adult I find it's not that bad of a show myself. Plus I know word for word the songs from the movies... No shame and no judgement. Hehe*
If you are interested, you are welcome to message me, so we could discuss further.
Just... You'd have to give me some slack a bit. ^^; It's be my first commission type thing.
Draw up a sketch and get me a quote on it. I'm thinking about 4 inches high ought to be about right.