Re: Which is stronger? Air or gravitational force?

You are spot on. The bigger the piece of paper, the larger the surface 
area, the more air molecules beneath it and the greater the resistance 
from the air to its downward passage.

A good trick to test your friends with is to hold out a playing card 
horizontally about half a meter to a meter over a bin and bet them a piece 
of candy that they can't drop the card so it goes into the bin. They will 
copy you and as long as the cards are held flat and dropped horizontally 
the resistance from the air means they will fall unevenly and miss the 
bin. Once you have enough candy - show them how to do it - turn the card 
through 90 degrees and drop it vertically, minimising its surface area and 
therefore the air resistance means it will go straight into the bin.

nb practice this first as you will need to get the drop height correct for 
size of playing card and size of bin.