"Well... a sturdy wooden box would do, right? A kinda big one, like..."

Can he mimic dimensions in front of him? Tidus attempts to put his hands out, to visualise it in front of him, but it's difficult when the idea is fresh, and he isn't sure exactly about the width.

But the height though, he makes somewhere in line with his waist.

"Something about this high should do, or, a little lower." Either way- "Then if you paint the lid one colour, that's easy to remember!"