TL;DR: Rainbow High dolls are going to be under a NEW size category: Petite Curvy
All the dolly details:
When I sort my dolls into sizes I pay less attention to their measurements as their overall shape. Humans generally fall into the same shape. No, we're not all the same shape obviously but if you're 6' or 5' your various bits all falling into the same area relative to each other so more attention is paid to your girth measurements like bust, waist, hip. This method doesn't work so great with dolls.
The first dolls I compared Rainbow High dolls to were other curvy dolls:
The horizontal red lines are aligned to various parts of the Rainbow High doll's anatomy (shoulder, bust, waist, hip, seat, knee, ankle) to show where other dolls do/do not line up.
Measurements wise, she's pretty similar in bust/waist/hip to Curvy Barbie and Squirrel Girl, but these are girth measurements. With the same hip height, where Curvy Barbie's smallest point is (waist) Rainbow High's bust is. So where Curvy is going in, Rainbow is going out. Squirrel girl is in between, with a slightly longer torso than Rainbow and slightly shorter torso than Curvy and can share clothes with both OK but not great, while Curvy and Rainbow have mostly problems sharing.
If Rainbow had a similar body shape, she would have measurements proportionately smaller at the same % that she is shorter. You can't just enlarge the pattern for the torso height because then she would be significantly larger in bust/waist/hip.
Let's try another one:
Here we have a lot more stuff lining up! Shoulders and busts are generally in the same place and the same size, seats are in the same place, but waist is in a different location (higher up), hips are much bigger and thighs are much bigger.
I pulled out some of my fit-then-flare patterns that are only fitted at the bust and shoulders like Tea Gowns and Shirr Romance to see if these clothes could fit the Rainbow high dolls. Eh. Technically yes, but aesthetically very much no. While the skirts stretch over the hips fine, they stretch over the hips to the point where the gathers become un-gathered. There's already a wide hip variation in the Petite size to accomodate the difference between the hippy pure neemo, Obitsu 24, and Hasbro Disney Descendants & Disney Princesses lines from narrower hipped Licca, Neo Blythe and Skipper dolls, but for those dolls the waist is about the same size, closer to the same location, and the thighs are the same size.
Finally, I did a mockup of the 3 current MGA fashion doll lines next to each other:
For this one I didn't do reference lines, instead I superimposed the OMG doll's torso over the other two dolls.
I have sorted the OMG and Pom dolls into the same size (Chibi) with 2 different torso variations because despite being way different heights, they have similar thighs, hips, bust, shoulders, and those things all line up at the same heights. The major difference is waist, which is minor enough that most patterns can do both dolls with a cut adjustment.
Rainbow High has roughly the same shoulder and hip as the other two dolls so you do see some sharing of things that only fit at those locations, like flared skirts that sit across the hips. Stuff that fits at more than 1 point, like a sheath dress or even a blouse, doesn't match up to be the right #s in the right places.
So the final verdict? Petite Curvy. Petite for shorter than the average 1/6 fashion doll, and Curvy for, well, curvy. Behold, the beginning of a new doll pattern size!
You can check out the new pattern size here. Obviously not a lot in it yet because it's new and it takes me a while to make patterns, but if Rainbow High is your thing, give it a bookmark and check back to watch it grow :)