Four questions to ask yourself while you start brainstorming ideas are:
What are the exaggerated characteristics of my hero/heroine?
What is the setting of my story?
What problem does my hero/heroine face?
How will that problem be solved?
When you can answer the above questions, you are ready to start your tall tale!
Here is an example of a famous tall tale...

Web Quest Steps
WEB QUEST INTRODUCTION
Step 2: PROCESS
Step 3: EVALUATION
Step 4: CONCLUSION