The value of pull-down resistor depends on the type of gate that follows the tri-state 74126 ..
For example, the amount of I[OL] current in a "standard TTL " is 1.6 mA ..
The biggest pull-down resistor you could use would be 0.8V/1.6 mA or 500 ohms ..
A larger resistor would allow the input to exist in the no-mans land between 0.8V and 2.0V ..
In case of the TTL-LS family the I[ol] current is lower, somewhere around 0.4mA, so the pull-down resistor can be 0.8V/0.4mA= 2k ..
And so on ..
Once you have a pull-down resistor in place, what happens when the level has to rise to the logic “1” ? Have you consider that?
Rgds,
IanP