(c) David Fiedler
Chain rings and sprockets are referred to by teeth count. The more teeth in the front, the bigger the gear and the harder it is to pedal and the faster you go. In the back, the smaller gears with fewer teeth are the ones that make you go fastest.
The 42/52 combination on this bike describes something like the medium and large chain rings up front by your pedals that you'll find on the typical 24 or 27 speed bicycle. The sprockets on the back of a road bike will typically range in tooth count from the low teens up to the 30s.
Your crank arms are what hold your pedals on, and on road bikes are usually between 165 and 175 mm. For a big guy like me who rides a larger frame bike, the longer crank arms are consistent with the overall size and dimensions of the bicycle.