This is a sort of informal mini-HIG for spacing in dialogs that we try to use in Xfce (well, that we're now starting to use). The new (4.6) settings dialogs use these values for the most part.

• Toplevel GtkContainer::border-width = 6
• Notebook tab GtkContainer::border-width = 12
• GtkVBox::spacing = 6
• GtkHBox::spacing = 12
• Internal GtkBox::border-width = 6
• GtkButtonBox::spacing = 12 (I'm not sure about this one)

• GtkTable::border-width = 0
• GtkTable::column-spacing = 12
• GtkTable::row-spacing = 6

• Indentation: use GtkAlignment, set:
  • padding-top = 0
  • padding-bottom = 6
  • padding-left = 12
  • padding-right = 0
  • border-width = 0
• For situations where the GtkAlignment is the first child of a GtkFrame, having no top padding looks a bit weird. In those cases, I set border-width to 6, padding-bottom to 0, and padding-left to 6. Should probably just use padding-top = 6, though, to avoid extra padding on the right.