Unless the series would actually include scenes from NS (though thinking about it, if Moiraine has a larger role in the series than in the books as I've seen suggested somewhere, showing flashbacks to NS might not be out of the question), does it matter if Moiraine voiced her height complex out loud at the time or not? The point is that it's clearly had a significant impact on her life and her personality, so making her tall does represent a more than purely cosmetic change.

And even aside from her private personal complexes, the books (and book covers) repeatedly dwell on the seeming incongruity of the small, delicate-looking woman wielding awesome power (at least initially, before most main characters start surpassing her in the Power) and having such a powerful will and strength of character. Rather like Yoda and his famous line about people who judge him by his size, to take a parallel from TV/movies.

Stepping away from that seems to be one indication that they're going to take a quite different approach to the beginning of the series. Which, you know, they probably had to, I'm not saying I'm upset about the casting. Just that I do think it's significant in a way.