The original caption on the RCT website was "Marie", but I disagree. One look in the face and I am 100% positive this is Tatiana. (furthermore RCT often gets the descriptions wrong - they even managed to identify Marie Fyodorovna and Queen Alexandra as Xenia and Victoria!). I don´t think she is too small either. First of all she is leaning back, second of all here is a comparisn photo from the same day:
Alexei and Tatiana have the same height difference in both pictures.
I rarely rely on such captions since they're almost always wrong. I was going simply on appearances.
As for the height difference in the two photos, they're actually not the same. In the photo in question, Alexei is bending forward slightly, making him appear shorter. In the other, he is standing straight. If he *were* standing straight in the first one, he would come up too high up on her chest for it to be Tatiana. Also, you can see that Alexei's hands reach the hem of the girl's dress. The same is true with Alexei and Marie in the photo below. Tatiana's hem, by comparison, is considerably lower. In my opinion, it is Marie.