She is 5’8″ or 172.5cm
36-28-36 (80-65-80)
34D
I try to use pictures that have not been in a magazine or part of a photo shoot. Normally those images are heavily photo-shopped and are not actually what the person looks like. Pictures like this are probably more accurate.
For example, look at these two pictures of her in bikinis (bikinis are an easy way to actually see the shape someone has - clothes can skew it significantly).
(Images from http://bumpshack.com/wp-content/uploads/2010/01/katharine-mcphee-bikini-shape-magazine-photos.jpg and from http://www.gotceleb.com/katharine-mcphee-bikini-candids-in-santa-barbara-2010-07-17.html/katharine-mcphee-bikini-candids-in-santa-barbara-23 respectively)
The first one is a magazine cover, and the second one is her setting up for some sort of photo. You can see that on the magazine cover that her waist to hip ratio is significantly smaller. Not real, people.
That's why I take what I see in magazine and photo shoots with a grain or 60 of salt.
Back on topic. As you can see, she has a wider waist in relation to her hips. Her shoulders are about as wide as her hips, which aren't very broad. Also, her legs are relatively shapely and slender, which is also a common feature of having a straighter body shape.
And yet, she's still considered to be attractive in general.