The pressure on the glass only changes if you increase the vertical volume of water behind the glasses foot print. if the volume of water is the same or less there will be equal to or less the amount of force.
That is why a 1 foot tall tank can have 8mm glass and why a 2 foot tall tank would want 20mm (measurements arn't perfect just ball parks.)
The only time where length is an attributing factor in the strength is where bowing would occur, because the tank is sealed every foot, as long as its supported as well, there shouldn't be an issue with bowing or the glass being stressed.