Why does the length of the wire matter to the value of the pull up resistors?
You are absolutely correct, a cable has parallel conductors with isolation in between, and that is a capacitor.
For example CAT5 cable has about 52 pF of capacitance per meter, and the longer the wire is, the more there is capacitance between a data and ground wires, and it takes longer for the capacitance to charge and discharge, given a certain current that is used to drive the wire.
As the DHT11 sensor use open drain communication, it means that it can only pull the wire down to ground actively, or let it float, as it cannot push the wire high. Therefore, the resistor is the only device that can bring the bus voltage high, by charging the wire capacitances.
So with longer cable, there is more capacitance, and to charge it within a certain time that is required to meet the high and low timing of the communication protocol, the resistance value must be low enough to charge the capacitance fast enough.
The formula is the same than for any RC system that charges exponentially. You have the time it needs to rise from certain logic low voltage to certain logic high voltage, the capacitance, and resistance.
But as the DHT11 datasheet does not give any real voltage or current specifications for it's data IO pin, there are no values to calculate anything. A real datasheet would give these values, so that minimum resistance value can be calculated from the maximum current the IO pin is able to sink, and then the maximum resistance value is determined by the capacitance of the bus, so that the signal rise time specification for the protocol is met.