It should be way bigger than that size at 3.5 weeks old.
Autoflowers are just that. They flower automatically. Usually within 3 to 4 weeks maybe 5 weeks if your lucky. If that was my plant, i would start another seed or 2.
But lets gets some more info.
What lights, nutrients, soil, grow environment etc.
You'll always get some stretch once you hit the 2nd week of flower or so, so don't worry too much. Also, you can have a viable plant that's only 14" tall, as long as you don't have your heart set on 'about a pound'. Remember it's a learning journey, it'll take a while to get it all figured out.