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.
This led to a proverb in Essex, reported in Thomas Fielding, Select Proverbs of All Nations: Illustrated with Notes and Comments (1824):
They may claim the bacon at Dunmow.—Essex.
Alluding to the well-known custom, instituted in the manor of Little Dunmow in Essex, by Lord Fitzwalter, who lived in the reign of Henry III.; which was, that any wedded couple, who, after being married a year and a day, would come to the priory, and kneeling on two sharp-pointed stones, before the prior and convent, swear that , during that time, they had neither repented of their bargain, nor had any dissension, should have a gammon of bacon. The record mentions several persons who claimed and received it; the last I find mentioned is, A. D. 1764, when Mr. and Mrs. Liddal, of the Green Dragon, Harrowgate, took the flitch of bacon oath. The custom ceased either for want of bacon or of claimants.
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).