What was said? We really have to “size up the bread box” very quickly to know what we are dealing with.
Did someone really say that? Yes – twice! In the same meeting when discussing a new project and not being 100% sure about the project scope, amount of data we had to process, etc.
What does it mean? It basically means that we don’t know the size of the item in question (the project itself in the case above) and an attempt to gauge the size by comparing it to a “standard” breadbox.
Origin: Apparently, the more common phrase is “Is it bigger than a breadbox?” when trying to “size up” an unknown object (in the case above, a project). On January 18, 1953 on an American game show “What’s My Line?” panelist Steve Allen originally asked the question when trying to guess the occupation of a mystery guess and was in specific reference to a product invented/produced by the guest. The phrase became very popular and a comic refrain on the long-running show that he later wrote a book titled “Bigger Than a Breadbox.”
The expression dates back a littler further to 1940 and was used more to describe an item with “no larger than a breadbox.”
For those unfamiliar with a “breadbox” – these were actual boxes (typically wood or metal) that sat on your counter top where you would store bread to keep it fresh and away from pests and mold (this was a time when there were no refrigerators to keep items fresh). A “standard” breadbox could fit about two average size bread loaves (16” wide, 8” tall and 9” deep). According to the internet… breadboxes are making a comeback with popular retailers like William Sonoma, Crate & Barrel, Amazon and more selling them.
HOT Idioms 
My family had a breadbox (made with mostly plastic, btw) in the 1950s and into the 60s, when we certainly had a refrigerator! The bread was not sealed inside a a wrapper bag with a twist-tie, and would not do so well in the relatively wet environment inside a fridge.