It really depend on the speed of link in your applications. You can check the pulse waveform to see the rise edge before determing the values. Some common value will be 1K to 10K ohms.
If you have oscilloscope you can see which resistor works fine (changing resistors you change edges of i2c signal). I use 4k7 for 100k transfer, but in some projects with PCF8583 I used 1k to get perfect square signals on i2c link.
It also depends on how many devices you have in that bus... I'll remember that Philips has some formulas to calculate resistor values. And don't forget series resistors from each device in bus; 100 - 150R fits most cases.