Totally Lyra

Yeah. I like that they did that in Past Sins and explained it really well. Though I guess that explanation couldn't work for the actual show, since there's no shreds of Nightmare Moon and no Nyx. I'm disappointed that we didn't get a chance to see more of Luna. Granted, if you look at my five favorite ponies, three of them are Nyx, Nightmare Moon, and Luna, so I'm a bit biased.

While I do like Luna, she's isn't my favourite princess since she doesn't appear a lot in the series. I really feel that the show could have placed more on Celestia, Luna and Discord. 

The shreds of Nightmare Moon were seen in the show itself in Season 1 Episode 2, but they were never mentioned again.

I would like to think that an alicorn's size is based on maturity. That wouldn't be too hard to put into the show and it doesn't need to involve the shreds. They could have just shown Luna getting more mature as the show went on, and by the end she should have grown to Celestia's size. 

There's a lot of things they could've done better, but I won't complain... too much. (like putting in Nyx, why didn't you make Nyx canon Hasbro? )

*cough* Ahem... ignore that.