Z.4.1- A player's hand is where they keep cards which have been drawn but are not yet played. [CompRules 1999/04/23]
Z.4.2- The number of cards in a player's hand is public information. All players have the right to know this number. But they do not get to know what cards you hold (unless a spell or ability says otherwise). [CompRules 1999/04/23]
Z.4.3- Each player has a maximum hand size. This is normally 7 cards. A player's maximum hand size is only checked at the beginning their cleanup step (see Rule P.13). Players may have any number of cards in their hand at other times. [CompRules 1999/04/23]
Z.4.4- Spell cards being announced are considered to not be in your hand for purposes of target selection, sacrifices, and other things related to announcing (see Rule T.4.2) the spell. [D'Angelo 1999/05/01]
Z.4.Ruling.1- You always get to know what cards an opponent sees when they look at cards from your hand. [D'Angelo 1997/02/10]
Z.4.Ruling.2- You are allowed to show cards in your hand to other players. [Barclay 1999/09/07]
Z.4.Ruling.3- Your hand is not ordered. You can shuffle or otherwise change the order of the cards in your hand at any time. [DeLaney 2000/01/22]