Question

If gravity is negligible then what effect it does on a person height.

Answer 1

As others have noted, the acceleration of gravity decreases proportional to the square of the distance to the center of the earth.  There are smaller effects from large local masses and the rotation of the earth, but for small changes in height on the surface of the earth, the difference in g is a decrease of about 3 micrometers per second squared for every 1 meter of height.  

For Mohamed Anas's example, a height of 40km would result in a decrease in g of (40km)(3 x 10^(-6) m)/s^2/(1m), or .12 m/s^2, very close to his calculations.

This is calculated from the formula  by taking the derivative.

Answer 2

My physics teacher told us (while teaching the topic gravitation) that gravity has a effect on height of human body, he gave us following examples:

The height of astronauts after coming back to earth is found to be increased slightly.
so the height of the person increases