I've found that for leggings/pants, 12m baby clothes fit her best. The leggings are stretchy so they fit over her legs, and usually end just below her knees. The pink cutoff "shorts" she's wearing are actually 12m jean pants. The waistbands sit on her hips, which is good because her waist is TINY and any further up there is serious gapping. I tried 18m size, but there is extra room in the butt to accommodate diapers so they don't work, either, as too low and they tend to bag and too high there is gapping. lol
Oh, I also bought two pairs of stretchy newborn-sized pants. They make fantastic shorts for Trinity.
Shirts, she also fits 12m but I think she could go up to 18m and be fine. The polka dot blouse is actually a dress, I think. Works just as well as a shirt. The flowered dress and striped shirt are actually toddler size. 2 or 3T, I think. They work if you want a baggy/oversized look.
The pink boots are size 5T, but she can wear a 4T for a snugger fit. 5T leaves room for socks/thicker stockings.
That's interesting about the My Size Barbie. I don't think I've seen one of those since I was a kid. I'll have to look up measurements and compare. It'd be interesting if she could wear their clothes. Although, I've found doll clothes tends to be more expensive than people clothes as a general rule. I think I spent less money on all those outfits from Kohls and Walmart than I did on that white gothic dress from Dollmore. lol
Oh, the first white dress she's wearing is actually a baby's Christening/baptism dress (and bonnet) I got off ebay. Size 12m, extra long skirt so it actually ends just at her ankles (it'd be uber long on an actual baby). Ties in the back to cinch the waist. Works very well for an old-fashioned, Victorian look. I want to find similar dresses in maybe some different colors.