Project 2003 automatic resource/task timeline adjustment

Hello, I am using project 2003 and am trying to get tasks to automatically come up with appropriate time frames based on resource max units and task timeframe. For example: I have a resource set to 50% max units. I create a 40 hour tasks, and assign the resource to the task. Based upon 50% max units, the resource can only spend 4 hours per day on the task, meaning the task should take 10 days to complete as opposed to the 5. I can create a calendar for each resource, limiting daily work hours to 4, but that seems to be quite a bit of work. Any assistance is greatly appreciated.