When you attack a Slime, either spawned naturally or through spawn eggs, The size of the Slime doesn't affect the health of the Slime. All Slimes I encountered take three hits with an unenchanted diamond sword to kill, or two hits if one of those is a critical hit. The sword itself hasn't had it's damage changed (tested by attacking Zombies and Skeletons)

Calculating the damage from 3 diamond sword hits (21 damage) or one regular hit and a critical hit (17,5 damage), this means all Slimes have between 7 and 10.5 hearts of health, regardless of size.

Update by 576874

As of 1.0.0.0 (beta build 3)

Tiny slimes have 0.5 health points - tested by punching the slime with a empty hand (+0.5 attack damage)., one hit kill

Small slimes have 4 health points - tested with a wood sword (+4 attack damage), one hit kill

Big slimes have 14 health points, when they should have 16 health points - tested with 2 diamond sword strikes (14 attack points). This method kills the big slime.

Slime Health Source