" I’ve got a rasher of bacon somewhere—[Feeling in his pockets]—I’ve the most distinct and vivid recollection of having purchased a rasher of bacon—Oh, here it is—[Produces it, wrapped in paper, and places it on table.]—and a penny roll."

I quote from the Project Gutenberg version. Whilst it's inconclusive, the presence of the penny roll would suggest a bacon sandwich & therefore a rasher much as we know it (perhaps a bit thicker than nowadays), rather than a gammon steak sized thing. Either way, I've never come across rasher being used in the way Mrrzy suggests. The only other quantity I can recall is a side of bacon (not the same thing as side bacon!), which is a salted & cured longitudinal half of a pig with the legs and shoulders removed (so not bacon in the regular sense).

PedantsRUs! ;-)