It is on the same page you use to turn off Sys Restore.
Start/programs/accessories/syste, tools/system restore/ In the available drives box, click Settings and slide the bar to the left. I have mine set for 1% or 200MB. That is enough for several restore points and more than I need.
Restore points are like old newspapers stacked up on your HD. In practice, you will never go more than one or two points back when restoring your 'puter. Why would you want a couple (or more) GIGs of old newspapers laying around?