Does the Enlarge/Reduce spell affect the Stone Golem? I had a player try to cast it on one, but I countered it because of the immutable form ability saying that counts as trying to change the form.
Enlarge/Reduce does not affect the stone golem.
Enlarge/Reduce (PHB 237) is a transmutation spell that changes (alters) the properties of a target creature, the target creature Stone Golem (MM 167, MM 170) is immune to that effect.
Immutable Form (MM 170) prevents the alteration
Immutable Form. The golem is immune to any spell or effect that would alter its form.
caused by the transmutation spell (PHB 203):
Transmutation spells change the properties of a creature, object, or environment.
Analytical: All transmutation spells alter the properties of the affected entity. Enlarge/Reduce is a transmutation spell. Enlarge/Reduce changes, among other effects, the size and weight. Changes of size and weight are changes of the original form. These changes of size and weight are alterations of form caused by a spell. The Stone golem is immune to alterations effects and spells that change their form. The Stone Golem is immune to the spell Enlarge/Reduce.