on the back of a postage stamp
In a very small space. Used to indicate an extremely limited or meager amount of knowledge about something. Why did they sit me next to a poetry professor at the wedding? Everything I know about poetry could fit on the back of a postage stamp! No one takes him seriously; he could write what he knows about socioeconomics on the back of a postage stamp.
on the back of a postage stamp
If you say that everything someone knows about a subject could be written on the back of a postage stamp, you mean they know very little about it. What she knew about children would have fitted on the back of a postage stamp.