Kids measurements can be quite confusing when it comes to patterns can’t they? I haven’t made this dress for my daughter – she grew out of the sizes too quickly so I have to enlarge the pattern to make her one but I did make the dress in the smallest size just to see how the pattern went.
If your little girl’s measurements fall into the 2T size but she is tall then it would be fine to make the 2T with a 3T length. However, if her measurements are at the top end of the 2T size, then I would be inclined to make her the 3T. She will be able to wear it for a while that way and you could also get a feel for how it will fit on her.
Both my children are very tall and I’m forced to sized just about everything up for my daughter because of this. But her measurements fall at the top end of size 10 so the 12 is a better fit for some reason.
If your little girl doesn’t like tight things then I would go for the bigger size. If you are really unsure, do a toile first to check and go with that.
Hope this helps.
Tamara