 The nucleus is about 10 to the minus 13 centimeters across. So you see that it is much smaller than an atom. So the nucleus is extremely tiny. Now the question is, how small is a string? A string is 10 to the minus 33 centimeters across. That is called the Planck length. We think that the Planck length is the ultimate length. That is, you cannot get smaller than the Planck length.