How do I work out what resistor to use?
The Pi has 3.3 volts, from an internal regulator, available on the GPIO connector - you can use that as a 3.3 V source for your pull-up resistors.

Using a resistor to produce a voltage drop is not recommended, as the voltage across the resistor will depend on the current drawn through the resistor.

The relationship between voltage drop, current, and resistance is known as Ohm's Law: V = IR, where V = voltage, I = current in Amps, and R = resistance in ohms.