The Header Sidebar is intended for ads of size 720×90. Other widget areas can support any type of content. Since the Boxes and Footer Sidebar displays in 3 or 4 columns it’s a good idea to display a number of widgets that’s evenly dividable by the number of columns. The Left and Right widgets will span half of the sidebar area while the Top and Bottom ones will span full width. The Wide and Footer Wide sidebars will display widgets in on column.