The 780 Classified is awesome, but if you don't plan to do some heavy overclocking, then don't bother with it. It's meant to be taken above Kepler's 1.21 limit set by Nvidia and can go up all the way to 1.5 volts. However, 1.3-35 is the highest that would keep within temps limits while water cooling. It's definitely a great card and I definitely recommend it if you plan on doing some serious overclocking.
There are a few factors to consider when getting a 780 Classifed. Will you water cool? If not, then you'd most likely be better off just getting the SC ACX from EVGA. The card really shines underwater and cannot wait to get my loop in.
Can your PSU handle heavily overclocked Classifieds? At 1.35 volts and an overclocked CPU, people have reported up to 1300 watts with two 780 Classifieds.
As far as the length, it is 11 inches long. The same length as a 690. It really depends on your case. With my Corsair Carbide 540 I can fit it and a 60mm rad with a few mm clearance between the two.