Pressure is defined as force per unit area. In the MKS system of unit of pressure is the Pascal - one Newton per square meter.
The big fellow at right has less effect when 'standing on toes' than his lighter friend because his shoes are in contact with a larger area of floor.
Atmospheric pressure is a little over 100 000 Pascals at sea level ..... or in more intuitively friendly units ten meters of water. Thirty meters under the sea a diver experiences a pressure to four atmospheres. Nitrogen slowly dissolves in his blood. If he stays down too long that can cause problems on surfacing. Divers have tables to to give them times that they can remain at different depths.
Example:
Knowing air pressure in Pascals at sea level and the radius of the Earth (6400 km) find the total mass of the atmosphere.