It's a decent size for an autoflower week 3. It'll grow a lot more during the next couple of week as it starts flowering.
- What you're calling flower, is "pre-flower", it's when the plant sex, showing whether it's male or female. Your plant is not flowering yet. ^_^
auto flowers start around 3 weeks to go in flower, yes you should be feeding the plant, looks healthy so far and the size is right for the age, give it a few weeks and i am sure the size will grow