There are LOTS of OSFM nappies out there - but not many of them are great for bubs that small, or if they are slender. I'd recommend something like the Pop-in because at least it has leg gussets if she has chicken legs

Oh - and my kids were long and slender, DD was in a small for about 6-7 months, and is now nearly 15 months and still in a medium. My DS is nearly 3 and fits into a medium.

So even though they are sized, look out for ones with a huge weight range (ie 5 -11kg) and you should get a good use out of it.