大 shows a person seen from the front, arms and legs spread wide — the "biggest possible" human posture. Where 人 is the side view of a walking person (small, narrow), 大 is the same person turned face-on with limbs extended (wide, big). The encoding takes a relative comparison and freezes it into a character. Identical across 繁體 / 新字体 / 简体.

It is one of the highest-frequency characters in the script and a productive radical for "big things": 太 (太い, fat / extremely — big with an extra dot for emphasis), 天 (sky/heaven — what is above the big person), 夫 (husband — adult man with hairpin), 央 (center), 奇 (strange — "great + can"), 奮 (vigor).

Mandarin: dà, falling 4th tone — the heavy drop matches the meaning. Less commonly dài (4th) in fixed compounds like 大夫 (dàifu, doctor). 大 is one of Mandarin's most overworked characters: 大学 (dàxué, university), 大家 (dàjiā, everyone), 大概 (dàgài, approximately), 大约 (dàyuē, roughly), 大小 (dàxiǎo, size), 长大 (zhǎngdà, to grow up), 伟大 (wěidà, great). The "big" sense extends into "approximate / general / overall" in many phrases.

Japanese: TWO on-readings, split by register. ダイ (dai) is more common — 大学 (daigaku, university), 大事 (daiji, important), 大部分 (daibubun, most). タイ (tai) appears in 大切 (taisetsu, important — note the same word maps to either tai-setsu or dai-setsu in different compounds), 大会 (taikai, convention). Kun-reading おお (ō) — 大きい (ōkii, big), 大阪 (Ōsaka, "big slope"), 大手 (ōte, big hand / major company).

Memory aid: a person with arms out, becoming bigger. The simplest mnemonic in the script.