3 meters or 4 meters, The range of medieval pikes(and by this I mean those of Flanders and Scotland) ranged from 6 to 8 "feet". And yes the Schiltron was not a circular formation, Pez's explanation is exactly why the schiltron couldn't have been a circle)

Concerning its use. At Bannockburn it was used defensively, as the English were foolishly attacking the well positioned Scots, who basically fought off the English who were attacking from two sides. The credit for the success at Bannockburn has to be also given to the non-pike men who were part of the Schiltron. Unlike the Phalanx the Schiltron consisted also of men with all sorts of polearms(as Sarcasm pointed out) and also dismounted men-at-arms(most of the knights and men-at-arms of Scotland dismounted to fought in the Schiltron).

So again the differences between Phalanx and Schiltron are significant.