It depends on how well matched the devices are. A good starting point would be something in the range of .33 to .51 Ohms. If they're completely random, you might consider going as high as 1 Ohm. Obviously, lower is better, but it's more money and more work. The better matched your MOSFETs are, the lower the resistance you can use.
When I do use them (the Rs), I tend to gravitate to ones that give around 0.6V at the rated MOSFET device SOA at zero crossing - so I can use the 0.6V to turn on the ( SF stage) protection transistor without the pull up or pull down R's influence.
Clear as mud.
i.e. 0.22 for a stage capable of 3A at zero cross.
They tend to smooth the Iq setting for high transconductance MOSFETS particularly in common source outputs, although losing output.
What kind of MOSFET's are you using?
If you use laterals (EXICON, Renesas/Hitachi), they have positive temperature coefficient starting already at very low currents. You may consider even not to use source resistors at all with those.
With verticals (all IRF etc.) it's another story though....