With CSS based layouts it is a common problem that columns can not be made of equal height. By Googling you may find may ways to solve this. Here I am going to show you an easy solution to achieve this comparing to the solution given by "Amanda" in her article "How to make columns of equal height in Blogger".
Assuming we have two columns "main-wrapper" and "sidebar-wrapper" . Below line will simply assign the height of main-wrapper to sidebar-wrapper .. which simply we want.
To add this script fist identify the column-ids and paste just above </title> tag in html editor. If you are not sure how to check for id's you can use Firebug a very popular add-in for Firefox for the same. By just hovering specific HTML gets selected in it. See the image below:
See the live sample here