How large should a 12 year old's penis be (soft and hard)?

Varies greatly: There is no one right answer for this, as everyone is different. Especially around the early teen years, there is a huge variety in size due to people starting puberty at different times. It is definitely something that most stress over during that time of life, but there is nothing that can be done to increase it, so it is important to try to realize that you are normal how you are.

What's the concern?: If the penis is really small or really large you should see a pediatrician. I'm not sure what your concern is since you didn't provide much information. Chances are that there is nothing wrong but seeing a doctor may allay any anxiety you have.