Squads (EU2012)
Four-Man Squad
The four-man squad is your default squad size.
The majority of your units this early will be rookies and therefore not have a class. Once you obtain sufficient ranked soldiers, you can start adjusting your squads for the missions you're embarking upon. The standard "one of each" is a typical approach here.
Five-Man Squad
The five-man squad is unlocked after purchasing the Squad Size I ability from the Officer Training School.
Six-Man Squad
The six-man squad is unlocked after purchasing the Squad Size II ability from the Officer Training School.
Achievement: Single-Man Squad
Clear a UFO crash site with one soldier on Classic or Impossible difficulty.
This mission is likely easiest towards the start of the game, when the aliens are not very strong. Heavy or Assault units would be most effective due to their high damage output and and their respective moving and firing abilities (Run & Gun or Bullet Swarm). Taking a medikit over a grenade will increase your longevity if you aren't killed outright, but the area of effect damage will be beneficial when trying to clear enemies more quickly.