"A Loaf" refers to a whole uncut bread, not a chunk. A loaf is the complete item, as removed from the oven. Bread in it's virgin state. (Western style bread I think, doesn't really work for unleavened breads eg. Pitta, Nan, Chappatti etc.).
Are you going to buy a wholemeal loaf or a granary loaf.
What a ready sliced white loaf? Shame on you!