The general halacha is that when bread is cooked (not baked), if the amount of bread cooked is larger than a k'zayis, hamotzei and bentching are required. If the bread being cooked is smaller than a k'zayis, one should make a mezonos and al hamichya (Shulchan Aruch 168:10). R' Bodner, in his sefer on Hilchos Brachos, writes that R' Shlomo Zalman and R' Elyashiv both pasken that deep frying has the same halacha as cooking.
This halacha plays a major role in determining the proper bracha on french toast. If one deep fries his french toast, if each piece of french toast is larger than a k'zayis, hamotzei and bentching are required, if each piece is smaller than a k'zayis, a mezonos and al hamichya are required.
If one makes french toast in a pan with a small amount of oil (not deep frying) even if the pieces are smaller than a k'zayis there is uncertainty as to whether this type of frying has the same halachos of cooking or not. In such instances, it is best to make a hamotzei on a k'zayis of challah which will then exempt the french toast from any subsequent bracha.
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