"not very tall" is correct and usable in written English.
You can use it to describe someone or something that is not as tall as would be expected. For example, "She was not very tall for her age."
"They're not very tall," he added.
My two (not very tall) brothers have both produced not very tall children.
They're not very tall, but they're well proportioned.
He is lean, not very tall, deliberate in his movements, a study in self-possession.
The woman was not very tall, but she did not have to stretch to see inside.
Most of them aren't very tall.
Roach isn't very tall but he pitches it up and has a good change of pace.
She wasn't very tall, and she was wearing a pink jacket, a light-blue skirt, and thin white stockings.
I actually prefer this style, because the regular panos aren't very tall (and, at 220 degrees, even wider than probably necessary).
June Steenkamp went back to Reeva Steenkamp's old high school in their home town of Port Elizabeth to talk to students, and read directly from the speech that touched on domestic abuse, family relationships, and even Reeva Steenkamp's struggles to break into the modeling industry because she wasn't very tall.
