In the tutorial, for example, a switch is wired in this fashion:
http://cq.cx/ladder-tutorial.pl
A 1k resistor is good. If you make the resistor much smaller, then a large current will flow; the circuit will work, but the resistor will generate a lot of heat when the switch is closed. (For example, with 100 ohms and Vdd = 5 V, the resistor burns (5 V)^2/(100 ohms) = 0.25 W, so it will be somewhat warm.)
If you make the resistor much larger, then stray currents, due to coupled noise or due to leakage through gunk on the printed circuit board, may significantly change the voltage at that node. So 1k is a reasonable tradeoff.