"big for watercolor" is a reference to the fact that most illustrators have a different idea of what a physically "big" art piece is than fine artists do, usually because of what medium(s) they work in

illustration majors: wow I JUST finished an 18x24 (inches) mixed media acrylic painting! it took a whole MONTH!

fine artist majors: (chuckling) I couldn't even FIT my last painting into my friends (pickup) TRUCK, I had a rent a Uhaul