It is preferable to use a loaf of bread that is at least the size of an olive.
But, since, according to most of the poskim, the mitzvah of lechem mishnah is rabbinical, it seems to me that it is possible to use a smaller loaf when
Two loaves of bread that stuck together while baking: Some poskim say that
they are considered two loaves even when they are stuck together and need not
be separated to fulfill the mitzvah of lechem mishnah. Since the intention
is to separate them, they are not considered to be one loaf, even while they are
still stuck together. This opinion provides a foundation in the Halachah for those
who bake a single loaf formed by combining twelve smaller loaves. Even while they
are still stuck together, the large loaf is, according to this, considered like
twelve separate loaves. But others hold that the two loaves that are stuck together
are not considered separate loaves until they are actually separated. Even though
it is evident on the loaves that they were separated from each other, once they
are separated, they are considered individual whole loaves because they were not
baked with the intention of their being parts of a single loaf. But if they were
placed in the oven with the intention that they stick together to form a single
loaf, it is unclear whether, even after being separated, they are considered separate
loaves. To avoid this doubt, they should be rebaked so that a crust forms where
one loaf was separated from another. Even though, after being rebaked, they are
considered separate loaves, they should not be sliced from the side which had
been connected to another loaf because there it already has the appearance of
When two loaves stick together in the course of baking and one of them is
cut, the remaining piece of the loaf that was cut should be removed from the whole
loaf to which it is attached before that loaf is used for lechem mishnah.
Otherwise, it would have the appearance of having already been cut.
If a person has only one loaf of bread, he should recite the blessing over
bread on that loaf. He should not break it into two. If a person has only slices
of bread, he should recite the blessing over two slices.