Target creature gets +2/+2 until end of turn. If you cast this spell during your main phase, that creature gets +4/+4 until end of turn instead.
If an effect copies Might of Old Krosa while it’s on the stack, the copy will give the creature only +2/+2, even if the original spell was cast during your main phase. This is because the copy is created on the stack and wasn’t cast at all.