I'm in a similar situation with my dds and really feel for you OP, it's so hard to know what to say.
Mine are both far too big for different reasons. One was recently diagnosed with PCOS and endometriosis, tries really hard, counts calories, nothing works, gets fed up and gives up. The other is more like OP'S DD - binging on crap to get through a really stressful uni period.
I cook healthy meals for them when they're here, but becoming young adults with access to loads of crap food (and skinny friends able to live on junk food) made both of them furious that they weren't skinny! At times they've both given up trying out of frustration, put on lots of weight, then got upset about it.
Both my dds talk to me a lot about their weight and ask advice. I try to separate the weight from the person, validating their feelings but reinforcing the positives about anything they're wearing/
hair/makeup/colour etc.
If they show me a really unflattering outfit, I do say something careful if it really looks awful. Eg "hmm, doesn't look comfortable at all and that's detracting from the look, but I do like the shape/length/colour."
I do reinforce that it doesn't matter to me what size they wear, I just want them to feel confident. (Particularly as they have an incredibly judgemental father and get endlessly nagged about their weight when they see him - which is becoming more infrequent as they vote with their feet.)
I've taken them shopping/looked up very stylish larger girls on socials etc (in response to wails of "NObody can look nice at this size..."). I've emphasised how crap womens sizing is - therefore labels don't matter, but if it's affecting health we can think about making changes. Etc.
Dd1 is ND and finds firmer boundaries helpful when she's in the mood. We talk about looking after herself and sometimes put a few outfits together for certain events so she feels prepared. When she wants to talk about it, I focus on general health and always style over size. God knows if it's the right approach but both talk to me a lot so that's positive.
Sorry, I'll shut up now but solidarity from me, it's a fucking minefield!