it depends on the units you are fighting with, and against. let's take two types of heavy infantry: romans and phalanxes.
for romans, it's short, thick, and sometimes double. your men are superior fighters and a small block is easily watched.
for phalanxes it's long and thin! if you're facing bad troops take this to the extreme: 3 men deep with hoplites, and 4 men with macedon style phalanxes. the troops behind that won't see action until the men in the front die. when facing better troops, you must make them a bit thicker, because you will lose troops and the men in the back get a purpose.
(extra note: on big phalanx vs. phalanx battles, another strategy works very well: put your troops in insanely thick blocks, with gaps between the units. the enemy phalanxes will be splitted by yours and will rout.)
Yep, it's true, having no sig is boring. But so is this one. Which makes my point... relatively pointless.
Can a point be a point when it is pointless?