He was so anxious that beads of sweat the size of peas formed on his head.
Tips
culture
斗 was a traditional grain measure (about 10 liters / a peck). 斗大 has split in two opposite directions: applied to text or sweat drops it means 'huge'; applied to a place it means 'tiny' (only a peck-sized speck on the map). Context tells you which.
memory
斗大的字 'peck-sized characters' = giant letters you can read from across the room. Picture imperial billboards painted in characters as big as a grain bucket.
