halacha - Challah Loaves on Shabbat have a minimum size?

In this article regarding the Shabbat Challah loaves, The Shmirat Shabbat KeHilchata (vol. II 55:5) is quoted as saying:

The loaves must each be at least the size of an olive

Normally, the size of the food being eaten does not matter when making a preceding blessing (see here, for example). The only requirement is that, in order to make an after-blessing, one must eat the required amount within a set time-period (see here, for example)

I don't have access to the Shmirat Shabbat KeHilchata, and I'm looking for sources and an explanation why the loaves used on Shabbat have a minimum size requirement.