10k Resistor Pull Up/Down Standard for 74 series chips
Unused high inputs for 74S and 74 series logic with multiple emitter inputs should be connected to Vcc through a resistor ( for protection of the input). The resistor can be calculated as follows (from these ancient scriptures): 



If you run the numbers you will see that 1K to 10K or even higher is fine for a pullup (but you'll get less EMI immunity with the higher values).  

For other logic families (74HC and other CMOS types, 74LS) the inputs can be connected directly to Vcc. (74LS is really DTL logic, there are no multiple emitter inputs). 

There is no reason not to connect any unused 74x input you want low directly to GND (use a 0 ohm resistor if you want to make it removable for some reason). Anything higher will reduce the noise immunity.

Of course you can also connect the unused input to an output that has the desired logic state. For example, grounding the input of an unused 74S04 inverter and using the output to drive up to the maximum fanout of inputs high



If it isn't really unused and you actually need a pull-down- a 250 ohm resistor will drop about 400mV at 1.6mA (numbers that mean something with the old  7400 series logic- 400mV is the maximum output voltage for 400mV noise immunity and 1.6mA is the maximum input current). 

The input current for 74LS logic is 1/4 as much (400uA), so 1K is the maximum pull-down for 74LS logic that still guarantees 400mV noise immunity. If you were doing some kind of funky diode logic this might be useful. 

Keep in mind that the input current is much less for a high input (40uA) vs a low input (-0.4/-1.6mA). 



On modern CMOS logic, the choice of pullup or pulldown resistor is largely a matter of noise immunity, speed (to charge input and stray capacitance) or in extreme low power situations, of leakage. Internal pullup/pulldown resistors are necessarily a trade-off and tend to be on the high side for high-EMI environments, so we often add lower value resistors in parallel. On the low side, drive capability comes into play. 

For example, for I2C bus pullups the bus capacitance vs. pullup resistor value can come into play.