大小 (dà xiǎo)

large and small; size; adults and children; consideration of seniority

Character breakdown

大 (dà) · 3 strokes

Depicts an adult man facing forward and standing erect. The meaning later shifted to "big".
小 (xiǎo) · 3 strokes

Depicts three small dots, representing the meaning "small". Originally the same character as 少. The small dots possibly represent grains of sand (沙).