Updated April 22, 2013
Constructed decks must contain a minimum of sixty cards. There is no maximum deck size; however, you must be able to shuffle your deck with no assistance. If a player wishes to use a sideboard, it must contain exactly fifteen cards.
With the exception of basic land cards, a player’s combined deck and sideboard may not contain more than four of any individual card, counted by its English card title equivalent. All cards named Plains, Island, Swamp, Mountain, and Forest are basic. (The five Ice Age block snow-covered lands -- Snow-Covered Plains, Snow-Covered Island, Snow-Covered Swamp, Snow-Covered Mountain, and Snow-Covered Forest -- are also basic lands. Note that snow-covered lands are permitted only in formats that allow expansions from the Ice Age block to be used.)
Legacy decks may consist of cards from all Magic card sets, any edition of the core set, and all special sets, supplements, and promotional printings released by Wizards of the Coast.
Cards from expansions and special sets (like From the Vault, Magic: The Gathering - Commander, Duel Decks, etc) are legal in the Legacy format on the date of release of the expansion or special set.
All promotional cards are legal in the Legacy format on the date of the release of the promotional card.
The following cards are banned in Legacy tournaments:
Official information about Constructed formats and the Legacy format can be found in the Magic: The Gathering Tournament Rules.