How did Sherlock know to use her measurements?

While Irene was very distracting and a great match for Sherlock in general this only hinders his deduction ability so much.

With Irene's display to Sherlock he can then continue in the back of his mind to try to figure out what series of numbers would be important enough to her to use as a combination. Counting on the fact that she wouldn't use a random set as the possibility to forget the numbers would be detrimental. Sherlock then has to figure out what is important to her? Her birthday? Her parents age at death? The locker combination she had in the 5th grade?

Ah, but Sherlock gets a peak at something most spectacular. The one thing that gives Adler power over Sherlock, even for a brief moment. Her body. Something that seems so important to her, that drives her livelihood, her profession, something that allows her to get what she wants when she wants it.

It is also ingenious in a way. A number not easily obtained without her knowledge (except in Sherlock's case of course). A number easily remembered by her, as well as a number most people don't use much in this era in addition to a number that fits perfectly as a combination to a safe.

Sherlock realizes this as a good possibility to the combination and gives it a whirl.

He knows that it's a six-digit code from the make of the safe.

I see from the make that it's a six-digit code.

He gets a number of clues from seeing how much the keys have been pressed.

You should always use gloves with these, you know. Heaviest oil deposit is always on the first key used - that's quite clearly a three - but after that the sequence is almost impossible to read.

It can't be your birthday - no disrespect, but clearly you were born in the 80s, and 8's barely used.

After that, Irene herself clues him in. First she teases him, as he was teasing her with the hiker case:

I'd tell you the code right now, but you know what? I already have. Think.

Then after the CIA agents burst in and threaten to shoot John, she genuinely wants him to guess it. She's been forbidden from speaking, but just as the CIA man is about to count to three, Sherlock looks at her and she looks significantly and meaningfully down at her own body.

At this point, he already knows it's a six-digit code starting with 3 and comprised predominantly - probably solely - of 2s, 3s and 4s, and he knows it's something she's "told" him already. With this information, that meaningful glance was enough for Sherlock to correctly deduce that the safe code is her measurements.

Knowing which keys were most pressed probably also helped him with getting the measurements right as well (guessing 34 instead of 36 for example) as, unlike cigarette ash, women's measurements at a glance doesn't really seem like the sort of thing Sherlock would have practice at calculating.

In solving a problem of this sort, the grand thing is to be able to reason backward. Sherlock is told to open the safe. He protests that he does not know the combination. Irene begins to say "Mr. Holmes doesn't ..." but is interrupted. Most people jump to the incorrect assumption that the end of her statement is "... know the code". (buzz sound) Wong. The sentences would have continued "... know where to look" a reference to there earlier conversation when Watson asks her to put something on. Perhaps a napkin. She says Watson does know where to look but suggests Holmes does not. When Holmes still does not clue in she glances down at her self, since he can not follow her logic she tries his. Once again referencing the earlier conversation when he says "fire exposes our priorities". In this case her priority is not to get shot thus needing Holmes to get the code.