R2 is a pull-up for when the opto is off. The value is dependent on switching speed and what you are driving with Vo. Lower values will give better edges for high speed; higher values will draw less current when the opto is on, sacrificing faster operation. Your load on Vo may have a min. current requirement that needs to be met, or if it's driving something like a LED or relay coil, you don't even need it. In short, there isn't enough info to give a range on the value.
You show C1 connected to Vo. That will limit the switching speed as well. Did you mean to connect that to Vcc as a bypass cap and not Vo?