Update for 1.4_00_01 -- if you bypassed Bukkit, you will most likely break.

src/main/java/org/bukkit/craftbukkit/entity/CraftSlime.java

@@ -25,6 +25,6 @@ public class CraftSlime extends CraftLivingEntity implements Slime {
     }
 
     public void setSize(int size) {
-        getHandle().e(size);
+        getHandle().setSize(size);
     }
 }