“the Bread of the Presence.” The Bread of the Presence was large cakes of bread that were in the Tabernacle and Temple (see commentary on Exod. 25:30).
“wafers.” The Hebrew refers to thin unleavened bread, matzah. It is not really a “wafer” in the modern sense of the word, but it is a small, baked unleavened flatbread.
“mixed with oil.” The translation “mixed with oil” is most likely what the Hebrew text and custom refer to, and many modern versions read that way. Some versions read “soaked” or “moistened,” which happened in the mixing, and some just refer to what is “mixed,” but the grain offering was mixed with oil (Lev. 2:4).
“and for all measurements of quantity and size.” David knew how important it was in a kingdom to have accurate weights and measures to ensure fair trade, and he officially put the Levites in charge of standardizing weights and measures. They were likely unofficially in charge of that earlier, but that is not clear. Early in history, weights and measures often varied from town to town. See commentary on Leviticus 19:35.