what kind of soil are you using? When you place it outside.. does it get direct sunlight? If so for how many days?

I'm guessing you have multiple problems occuring that are stunting your plants growth. You need to post this question in the plant problem forums and fill out the troubleshooting form for us to diagnose the problem more accurately.

To answer your initial question:

No one can tell you how big a plant should be at a certain again; just like people it will vary from individual to individual.. or in this case from plant to plant.