From the Calendar Availability tab in a fixed event, click the Add Availability button.
Set a start and end date and time by using the drop down selectors.
Add a length to the time blocks by selecting a time interval from the Length drop down box. This will determine how long each available time slot lasts.
Set the number of available attendees for each time slot in the limit field.
Choose to exclude or include weekends if your selected time spans a weekend.
Choose to include or exclude yourself and add internal participants by typing in the participant name field.