The cob should fill out the shuck in a well rounded, firm but not hard fashion assuming the corn has had all the nutrients and water it needs. The size of the ear and the kernels will vary from variety to variety. If the silk are "just starting to turn brown" I'd give it a couple more days but that it hard to say without actually seeing it.

So where is it DTM-wise and what size ears is your variety supposed to produce?

When inexperienced the best way may be to peel an ear and look at it.

Dave

I had to do just that, pick one and see. I picked one and saw that the kernels were too small. Waited about a week, still there were some more room for the kernels. And third time was the charm. I picked the fattest ones at the time. Then waited few days, and picked more, and repeated the process.

As the kernels fill out and the ears get heavy, they will start leaning away from the stalk ever-so-slightly. Then you can feel the difference between ripe ears, which are firm to hard, and underripe ones, which still have a cushion of silk between the husk and the cob.

This occurs about 21 days after full tassel.

Thank you all so much. They aren't really leaning yet, but I will watch for that. I'll probably go ahead and pick one and check. And Dave, I'm not sure how to find out what size they are supposed to be. I'm in southern Missouri, if that helps. My dirt is definitely lacking nutrients but I've put down some plant food and that seems to have helped.

I wait until the silk turns brown and emits dust when flipped with a finger.
At that point, I will fold back the shuck a bit and see how the ear is progressing. When I can get an ear to emit a milky white liquid when pierced with a fingernail, I pick.

Trial and error is the surest method. After a while you can tell by the way the ear feels in your hand.