Picture a small sewing thimble to gauge how much 'thimbleful' represents when considering amount. I added just a thimbleful of oil to the dressing.
Pro Tip 2/3
Not for Precision
'Thimbleful' implies an approximate amount, not meant for recipes or situations requiring precision. Just use a thimbleful of vanilla extract; the exact measure isn't crucial.
Pro Tip 3/3
Often Figurative
While it can refer to literal amounts, 'thimbleful' is frequently used in a figurative sense for liquids. Each guest received a thimbleful of the rare wine to taste.