If unsure of sizes, perhaps lingerie is not the best thing to buy. things like bras have to fit properly, or else they won't be worn. Especially if she is on the large busted side.
As TinglingTess said, how about going for something like a silk/ satin night gown with matching robe. Something glamourous and something that you find sexy. Something like that will be more a case of general dress size (easier to guess at) than the exact proprtions required for buying fitted lingerie. I have a lot of experience in bra buying being larger than average in the bust and smaller than average in the back (for my size elsewhere) and I wouldn't have a clue about guessing someone elses size from a picture or in real life, so I wouldn't risk guess work personally. You run the risk of offending her if you choose the wrong size!
Also, personally I'd go against the gift certificate idea too, since the whole idea is surely that you spent time and care choosing the gift. I received beautiful lingerie for Xmas, and I know the thrill was that he chose it alone, got the right size (because he'd asked me before hand 'just in case' he might decide to buy something like that) and had gift wrapped it in a big box with layers of tissue paper and a ribbons. Those things are often important to us girls....it's all in the presentation.
If you have that kind of relationship with her sister (if she has one) or her Mum, then I'd ask. I know that mine would be more than willing to help out if they were asked and neither would think it was strange.
Oh and Patchouli and Ylang Ylang are often scents used for 'romantic' candles, although personally I think they're an acquired taste!