I'm more interested in how accurately the words describe the situation than in what words are used. A description of a '15-inch penis' can be perfectly okay in a story about dinosaur sex, it's really stretching it if they're human adults, and it just isn't imaginable when they're minors (and yes, I have seen stories of the last category). People really should learn basic anatomy. A six inch penis is pretty decently sized for an adult, especially considering the size of what it needs to fit into.
The aforementioned 'fuzzy clam' meaning vagina is pretty good example of bad use of non-concrete language. The author was probably combining the tightness of a clam and the pubic hair, but it came off to me and the previous poster as a clam (the bivalve) with hair on/in it. A good description or descriptive word either has only one reasonable way of being interpreted, or is a double entendre.
Units of measure can be very good in that they can be far more precise than descriptives (though I would stay away from dick measurements - it's way overused). The problem is that preciseness puts an additional burden on the author. Exaggeration with units is usually not okay (unless _so_ farfetched as to be obvious, eg., "I'm so hungry I could eat an elephant."). On the other hand, a good measurement can be very useful. "They hiked 15 kilometres" or "they hiked until midday" will give the reader a better idea of how much effort was involved than "they hiked a long time" or "they hiked and were tired."
Just be careful that the numbers you choose make sense and that they convey what you want to mean. If a character in your story is described as being 8ft6in, then you're going to need to be very careful in what sex acts they partake in. Some of them might be very awkward, especially if the woman is 4ft3in (half the height and 1/8th the weight assuming equal proportions). Likewise, that guy with the 21-inch dick (probably the longest length I've ever seen in an actual story) is going to have some serious fainting issues trying to supply enough blood to fill it (the volume is comparable to the blood in an average body), and I sure hope that the woman has a vagina extending well into her chest cavity (or the guy a very accomodating large intestine).