The optional size attribute defines a recommended size for visualization of an infinite drawable element such as plane, cylinder, axis, etc., or objects without explicit geometric parameters (e.g. coordinate system).
The optional transparency attribute defines the transparency property of a model entity.
Source
<xs:complexType name="DrawableBaseType" abstract="true"><xs:annotation><xs:documentation>The DrawableBaseType is the abstract base type for drawable elements. It contains data necessary for a model entity visualization.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="NodeWithIdBaseType"><xs:attributeGroup ref="AttrDrawable"/></xs:extension></xs:complexContent></xs:complexType>