In English, “bread” is uncountable. We say:

Do you want some bread? 

If you want to count bread, you have to use one of the following counters:

• a piece of bread can be any size
• a slice of bread is a flat piece that can be used for toast or for one side of a sandwich
• a loaf of bread is the “whole bread” - the bread as it looks when it comes out of the oven

We use “loaf” when we talk about buying bread:

I bought a loaf of bread, some milk, and some apples.