Since when is the "slab" a standard unit of measure?

Sign for "1/4 Slab Cake", Safeway, West Vancouver, BC 

I saw this sign at the bakery department of a Safeway store. The "1/4 Slab Cake" does look pretty, but how big is it?

This is one of the regular problem of businesses that assume that their internal language is understood by the customer. I don't know how big a slab is, and there is nothing to tell me how big the "1/4 Slab" is? How hard would it be to include a size? Imperial or metric? Or even better, both?