Disclaimer that if you have a C cup and above this dress may accentuate that A LOT. I know for some they say these dresses don't look like they do on the models but if you don't look like the model the dress will not look like that on YOU!
I am (usually) a top size 8 and a bottom size 10 - 34B/C for reference!
I wear a M in the Ophelia as this does not have smocking all the way around and there is a small invisible zipper on the side seam - so sizing to a small would make it hard to zip! I would say most people can size 1-2 above their Ellie size for this as the Ellie is EXTRMELY generous in sizing.
Ophelia runs a bit longer on me than the Athena and Norah Styles, definitely longer than the Vivi. It's about an inch and a half past my finger tips. A normal bra will not work with this dress but my plunge strapless bra does. However, I find I do not need a bra (as with almost all HHH dresses,) as the top is very supportive. You can make the dress more supportive by tightening the tie you have between the bust and/or by wearing the tie as an extra halter of sorts (HHH used to do this too, go back and look at old IG posts.)
Specifically regarding the Blue Peony Bouquet Cotton Sateen color - I was worried about it being sheer but it is quite thick - I can wear shorts underneath and they do not show through.