What were the typical dimensions of a WW2 US bomber formation?
The size of the bomber formations (bombers streams) varied significantly depending on the number of bombers , the time , location and other things. The most common figure quoted figure is around 70 miles long and  5-6 miles wide (for ~500-600 bombers). However, the formation's footprint depended on a number of factors ranging from the navigator's skill to the wind in that area.

The figures vary wildly. For example, from the book Mission to Berlin: The American Airmen Who Struck the Heart of Hitler's Reich:


  The stream of more than one thousand Eighth Air Force bombers, from one end to another was 360 miles long. ... on ... February 3, 1945, when the first flying fortresses reached Berlin, the last bomber was over the Zuiderzee in Holland.


This extreme length is a consequence of the situation late in the war, where allied air armada roamed European skies without bothering much about the Luftwaffe. The time spent over the target, on the other hand was quite small-


  ... each Fortress crew would spend only between thirty and sixty seconds over the center of Berlin itself...


Note that there was no such thing as precision bombing in WWII (except for some special cases), though the accuracy did improve over the war. Of course, some of this was due to the way the 'target' was designated- anywhere 1000' of the aiming point was good enough. From The United States Strategic Bombing Survey:


  Conventionally the air forces designated as "the target area" a circle having a radius of 1000 feet around the aiming point of attack. While accuracy improved during the war, Survey studies show that, in the over-all, only about 20% of the bombs aimed at precision targets fell within this target area. A peak accuracy of 70% was reached for the month of February 1945. 


Accuracy of RAF night bombing improved from ~20% in 1942 to > 90% in 1945. Of course, the British target was comparatively larger- anything within a 3 miles radius of the aiming point was a 'hit'.