In the speaking band descriptors (band 7), it mentions in the Lexical Resource section that you should "use some less common and idiomatic vocabulary and shows some awareness of style and collocation, with some inappropriate choices." Here are some less common and idiomatic vocabularies that have been used frequently in our daily life.
1. knee-high to a grasshopper
very small or short. (Usually used to describe children. *Typically: be ~; since someone was ~.)
E.g. I've known you since you were knee-high to a grasshopper.
My, how you've grown! The last time I saw you, you were knee-high to a grasshopper!
2. neck of the woods (informal)
area of the country
E.g. I'm surprised to see you in this neck of the woods. What brings you here?
There's no scenery like this in your neck of the woods, is there?
your neck of the woods
the area you come from or where you now live
E.g. If you're in our neck of the woods, we hope you'll come to see us.
Usage notes: often used in the form this neck of the woods.
E.g. It was a pretty small farm for this neck of the woods.
3. Not the sharpest tool in the shed / not the brightest bulb in the chandelier
A gentle way to say that someone is just unintelligent
E.g. Danny failed his third test this week.
Well, he's not the sharpest tool in the shed, you know.
4. rough around the edges (comparative rougher around the edges, superlative roughest around the edges)
in need of refinement; unsophisticated
E.g. His writing is appealing, but a bit rough around the edges.
5. act your age, not your shoe size
Stop behaving immaturely. Said to adults who are acting like overgrown children and to school-age children who are acting like overgrown toddlers
E.g. It's time for you to get serious and stop being silly. Grow up, pal! Act your age, not your shoe size!