Outlook's calendar supports multiple time scales, from 5 minutes to 1 hour. While you can't make your own interval, you can change the scale to one more suitable for your needs.
When you create a new appointment or meeting and are in the Day or Week view, the default length of the appointment is the same as the chosen interval.
When you create a new appointment in Monthly view or from another Outlook folder, the default appointment length is 30 minutes. This cannot be changed.
In the Day or Week view in Outlook's Calendar, right click on the column along the left side, that shows the hours of the day. By default the interval is set to 30 minutes.
Right-click anywhere on the hours of the day column and choose the hourly interval you want from the context menu.