大 is a pictograph of a person (人) with arms stretched wide, showing something big.
mistakes
The character is dà almost everywhere, but one common word breaks the pattern: 大夫, doctor, where 大 is read dài. The same two characters read dàfū are a rank of official in ancient China, so the reading picks the meaning.
Person figure underneath - body and legs viewed from the front. Strict etymology says 大 is one pictograph of a frontal-stance person, but visually it splits cleanly into 一 + 人.
Radical
BigKangxi #37
The 大 radical, a pictograph of a person with arms outstretched. Indexes characters about people, scale, or magnitude: 天, 太, 央, 夫, 头, 奇, 套, 奖. Common as the top component but a frequent semantic anchor wherever it appears.
