For an adult I would recommend 35 cm x 35 cm x 60 cm, it's not about overall volume but hight.

Good thing that you do not plan to Cohan because gargoyle should not be house together they are a lot more territorial and aggressive then crested.

Also remember if you get a young individual the enclosure will need to be smaller so it can find his food easily, in other words do not put a baby in an adult cage.