Merriam-Webster's Eleventh Collegiate Dictionary (2003) identifies three terms that may refer to a "huge flank of bacon": side, flitch, and gammon. Here are the relevant definitions of these words:
side n 1 b (2) a cut of meat including that about the ribs of one half of the body—used chiefly of smoked pork products
flitch n 1 : a side of cured meat; esp : a side of bacon
gammon n 2 chiefly Brit a : a side of bacon b : the lower end of a side of bacon
In contrast, MW defines a slab generally as "a thick piece or slice (as of stone, wood, or bread)" and doesn't indicate how much of a side/flitch/gammon of bacon might constitute a slab.
In his entry for Donmow (or Dunmow), Nathan Bailey, An Universal Etymological English Dictionary (1724) relates a fine custom of the place involving a flitch (or gammon) of bacon:
DONMOW, DUNMOW, a Priory is Essex, where there was a Custom, that any Person who had been married a Year and a Day, upon taking Oath before the Prior and Convent, that he had not repented of it in that Time, was intitled to a Gammon or Flitch of Bacon, which being delivered unto him, he was conducted out of Town with great Solemnity.
A detailed account of the Dunmow Flitch ceremony appears in George Monger, Marriage Customs of the World: An Encyclopedia of Dating Customs and Wedding (2013).