@Ranch
Well, taking this shot from Luna Eclipsed we can see that the top of Twiligh’s head goes up to Luna’s chin or thereabouts;
And taking this shot from Applebuck Season we can see that the top of Applejack’s head reaches about halfway up Big Mac’s head.
And from this shot we can see that (pre-alicorn) Twilight and Applejack are pretty much the same height:
I’d say Luna is slightly taller than Mac but not by a whole lot. And she probably also appears a bit taller than she actually is because of her horn and mane.