There are a couple of popular
signs for "tall."
A good general sign is made by placing your right index finger on the palm of your left hand
(if you are right handed) and moving
the finger upward a few inches. This version is a good one to use to describe trees,
buildings, or even people.
This next version is particularly good to use for describing people but not
good for describing buildings.
I've inflected this version to mean "very tall." You can do it
with less facial expression and not so high up and without the upward
eye-gaze to mean just "regular tall" rather than "very tall."
very-TALL (basketball player height or "giant")