What's the input resistance of an opto-isolator?
You need to look at how much input current your optoisolator needs, and then use a resistor that passes that amount of current when it has 48V across it.

Technically, you should first subtract the voltage drop of the optoisolator from the supply voltage, but in this case, the difference should be negligible.

For example, if your isolator needs 10 mA, you would select a resistor of 4700 &Omega;, which is the closest standard value to 48V / 0.01A = 4800 &Omega;. Note that this resistor will be dissipating 48V &times; 0.01A = 0.48W, so select a 1W resistor for good reliability.