If this value is too low for an active site, then performance problems (on any service: web, smtp, etc) may be apparent due to a bottleneck as requests are queued up.
Like the other memory settings, this option has no set in stone recommendation. How high you can go is a blend of your server box's capabilities, average load, etc.
However for a moderately busy site, start with 50. For a large site start with 100. Increase/decrease these values to find the best balance for your system.
Unlike the other memory usage settings, this value may be adjusted on the fly and will not cause your Web Crossing server to stop/restart.
We don't recommend arbitrarily maxing this setting out, but the highest value it will accept is 1,316,134,911