How small is small?
The physical casing surrounding a rotating machine that meets IP67 or 68 is going to impose some limitations on minimum volume because to meet the ingress protection rating requires that there is no effective air exchange between the exterior and interior of the casing. (Completely dustproof and waterproof to at least 1 m for 67, to more than 1m for 68). The purpose of the fan is to move air - which means it's going to need some diameter and length as well. When down in these size ranges, axial flow fans are the norm (in part because that helps to move air over the casing exterior, resulting in better convection cooling).