The Window Buttons panel plugin allows allows you to switch between application windows using buttons.

You can set a custom icon size in gtk-icon-sizes with the name panel-tasklist-menu. The default icon size is 16px.

.gtkrc-2.0

style "xfce-tasklist-style"
{
  # The maximum length of a button before the label ellipsizes.
  # When this value is set to -1 the button will expand to the
  # entire available space.
  XfceTasklist::max-button-length = 200
 
  # Ellipsizing used in the task list and overflow menu labels.
  XfceTasklist::ellipsize-mode = PANGO_ELLIPSIZE_END
 
  # Lucency of minimized icons. Valid values are between 0 (completely
  # hide the icon) and 100 (don't lighten the icon).
  XfceTasklist::minimized-icon-lucency = 50
 
  # Maximum number of characters in the menu label before it will
  # be ellipsized.
  XfceTasklist::menu-max-width-chars = 24
}
class "XfceTasklist" style "xfce-tasklist-style"

See the section Theming for more information.