In millions of starts, there will be strands stopping at every possible T along the way. ALL of the strands we make started at one exact position. Here's how we find out those fragment sizes. Gel electrophoresis can be used to separate the fragments by size and measure them. The dideoxy-C, for example, glows blue.
Smallest fragments are at the bottom, largest at the top.
The positions and spacing shows the relative sizes. Simply by scanning up the gel, we can see that we skip two, and then there's two more C's in a row. And so on, all the way up. We can see where all the C's are. The spacing between the bands isn't all that easy to figure out.