Selecting a head of iceberg lettuce or a bunch of celery or a one pound package of carrots? Weigh it, by hand first, and then on the nearest scale (you'll find one in the vegetable section of your local supermarket). Goes for anything that sells by the dozen or by the pack (as opposed to by the pound.)
What you'll find is that all lettuces are not alike; all bunches of celery are not alike; all one pound bags of carrots are not one pound . . . some weigh more, some weigh less. So trot over to that scale, weigh your vegetables and make sure you're getting the heaviest bunch of celery, the heaviest head of lettuce, the heaviest "pound" bag of carrots on the counter!