dpinion - 8:09 pm on Oct 13, 2010 (gmt 0)
Your post has me quite confused. When you say horizontal dotted line, I assume you mean to say vertical? From your code your div should be the same size as its <td> container(firebug confirms this).
You can vary the height of either div to expand it down:
<div style="border-right: 3px dotted black;">
Lots of content in here, with divs, etc...
Longer than Column 2
<div style="border-right: 3px dotted black;min-height:200px">
Some shorter content here
What exactly are you trying to do?