The following KB articles relate to this topic:
You can use layout blocks to manage a variety of content on your webpage. Layout Blocks are good for aligning content and efficiently using up window space. The rule of thumb with layout blocks is that the widths of the columns, when added together, must equal 12 units. With the three column layout block, the default column widths are exactly equal (4, 4, and 4).
The columns do not necessarily need to be equal widths, however. You do have the ability to change the widths after building the content block. For example, you could have one with a width of 5, one with a width of 3 and one with a width of 4 - it works as long as it equals 12.
It is also important to note that the information provided in these columns can be different, too. In the example below, the first column contains a regular text area, the second column contains an RSS feed, and the third column holds an image.
This first block contains only text via a text area content block. This means that anything allowed in a text area can be added to this region. For example: