Increasing altitude causes the air pressure to decrease. As you go higher in altitude, there are fewer air molecules above you, which results in lower air pressure.
As for air density, this also decreases with increasing altitude. At higher altitudes, the air becomes less dense because there are fewer molecules in a given volume of air.
The size of gas molecules remains constant regardless of altitude. Gas molecules have a specific size that does not change based on altitude.
The space between gas molecules actually increases as altitude increases. With lower air pressure at higher altitudes, the molecules are further apart from each other compared to lower altitudes where they are closer together.
