1.
big, large (in size)
Describes physical size. Used for objects, spaces, animals, and anything that can be measured by size. The opposite is 小さい (chiisai, small).
2.
loud (of sound/voice)
When describing voices or sounds, 大きい means loud. 大きい声 (ookii koe) means a loud voice.
大きい is an i-adjective. Before nouns, it can be used as 大きい or 大きな (ookina). 大きな is a special attributive form that can only be used directly before nouns (大きな家), while 大きい can be used both attributively and predicatively. Conjugation: 大きくない (negative), 大きかった (past).