How do I restrict a driver's pick-up radius size?

Go to DriversManage.
Click Modify next to the driver you want to update.
Scroll down to the Queue Settings section.
Find the Offer Job Radius setting.
Enter the maximum number of miles within which this driver should receive job offers.
Click Update to save your changes.

Once saved, the driver will no longer receive job offers outside of the specified radius.

Offer Job Radius setting under driver queue settings