The maximum Deck size is an unofficial term used to refer to the upper limit on the number of cards allowed in a player's Main, Extra, and Side Decks. Since its introduction in the Yu-Gi-Oh! 5D's series, the limit has been 60 cards for the Main Deck, and 15 for both the Extra and Side Decks.
Before its introduction, there were no upper limits to either the Main or Extra Deck. The Side Deck, however, was required to contain either exactly 15 or exactly 0 cards.