If you are talking about the width of a block, then look at "Advanced Features", then "Edit Style Sheet". Usually at the bottom are the settings for template width, right column width, and left column width. You can edit these, but save your email first and edit with caution. Make sure your right and left column widths add up to total template width. You could also do some tweaking in the html code view for each section you are editing in the email. But only do this if you understand html code.
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