The Kozii reservoir is 3.1" in diameter and 3” deep. The diameter is oblong, to allow water to flow around a 3.1” diameter bottle.
Kozii also has a narrow slot with extra width to accommodate flat, frozen breastmilk storage bags up to 3.8" wide. As such, Kozii will accommodate all bags and bottles found in the marketplace.
You can find a dimensioned drawing of Kozii's warming chamber at https://www.kiinde.com/pdf/kozii-inside-dimensions.PDF