From the Super Mario Wiki
The sprite of the giant ? block
Giant Question Blocks (or Giant ? Blocks) are blocks found in Super Mario Bros. 3 and its remakes. They are found mainly in the game's World 4 (Big Island), but can also be found in certain secret areas throughout the game. They contain regular power-ups in Giant Land, and usually multiple 1-Up Mushrooms or various Suits - such as the Tanooki, Frog, and Hammer Suits - in bonus areas.
Two Giant Question Blocks in Super Mario Galaxy 2
Giant Question Blocks are also found in Mega Mushroom Toad Houses in New Super Mario Bros., where they would give Mario or Luigi a Mega Mushroom.
In Mario & Luigi: Bowser's Inside Story, Broque Monsieur has a Giant Question Block in his collection, but it's unobtainable throughout the game.
Giant Question Blocks re-appear, albeit rarely, in New Super Mario Bros. Wii. They appeared in World 6-5 floating on water.
In Super Mario Galaxy 2, they appear in the Supermassive Galaxy. Mario and Luigi are unable to hit these blocks, and they are merely used as platforms throughout the level.
The Giant Question Blocks return in New Super Mario Bros. U as blocks in various Soda Jungle courses.