
Many magicians are led to believe that they need to have a brief of less than 64th of an inch (about half the width of the white border) when dealing seconds in order for the deal to be deceptive. But the truth is, the rhythm of the deal is more important for it to be deceptive.
The brief can be as big or small as you like it to be. It was said that the theory of the small brief was a rumour started to discourage people from learning the second deal. According to Vernon and Giobbi (of card college fame), the brief means nothing if your timing is off- it's more important to be smooth. And learn to sail the card gracefully. :-)