Java JaxB generation, How do I get a bigDecimal from my xsd?

I have an xsd annotation that I am trying to get to Marshal into a java object. I would like the java to end up with BigDecimal for its value. What do I enter in the xsd to make it do this? I am using an xjc ant task

<xjc schema="my.xsd" destdir="generated" header="false" extension="true" />

Here is the relevant xsd -

<complexType name="Size">
    <attribute name="height" type="BigDecimal"></attribute> <!-- this is wrong-->
</complexType>

I would like to end up with this for the generated class -

public class Size { 
@XmlAttribute(name = "height")
    protected BigDecimal height;
}