Well he'd still be using the notepad in combat for quicker and less potent constructs, He'd only have the large paper for missions where it was planned out, they'd be more potent as he'd have time to paint them rather than sketch them. The thicker the brush and therefore the lines, the more potent and capable of defense the drawing would be. And the to scale part isn't the issue. Artist wouldn't have paper the size of elephants, so some constructs clearly wouldn't be able to be actual size, but with enough prep-time it'd come pretty damn close. Think of it like Batman, Artist is at his best when given a situation and being allowed to prepare for it, though he is still capable when he cannot prepare, he just has to think on his feet with less resources to do it with.