To change the width of the sidebox columns, open your Admin and point your cursor at Configuration on the menu bar.
Choose Layout Settings from the dropdown menu.
Click the Box Width you wish to change and edit it, the width can be fixed (default is 150px), expressed as a percentage of the total page width or as a relative size (20em). You can also change the width of either column to suit your design.
For example: if you were using a fluid design and wanted the boxes to shrink and grow depending on the window size you would set the columns to 20% and the boxes to 100%.You can also turn off either column universally in the same section of the admin.