This is an expression that has almost gone completely stale. When I Googled “bigger than a…” the rest of the expression was the first suggestion, but that might be Google accounting for my age cohort’s known preferences, because the expression has very little relevance or resonance today. Nobody under 40 is likely to know what a breadbox is. Furthermore, “bigger than a breadbox” is not the most precise measurement in the world. Still, invitation to imagine a size was part of the expression’s charm in its erstwhile usage, just like someone used to be invited to “set for a [completely untimed] spell.”
The vague lack of specificity is telling and, sometimes, trying, however, when trying to explain the expression to those who haven’t heard it. How do you describe to someone who has never seen a breadbox just how, um, big it is?