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Go to ShoutBox Preferences in admin interface.
Set the Text Input width parameter to correspond to the width of the column.
Example: Let's say your left column is 160px wide and the module parameter Shout Input Text Type = Multi-Line Text Area. You would set the Text Area Columns parameter = 30 (note: this number indicates how many characters wide the entry box is)
If Shout Input Text Type = Single Line Text then you would set the Text Line Length = 30.
Hope this helps.