What siding material is available for a tiny house with metal framing?
Looking at older type mobiles, park models and even some of the newer tiny homes I have seen built.  A common out layer starts with a thin sheet of exterior plywood (really more like paneling or underlayment)  sizing between 1/8 and 1/4 inch thick.  Then skinned with tyvek or plastic and final covered with various exterior materials (aluminum, galvalume tin, fiberglass, or more wood).  With this sandwich method the cheaper wood reduces the thickness of the out material saving costs.  

The final outside depends on what your seeking for looks, for instance you can buy a Tuff-rib(tm) panel galvalume protected and painted or coated cheaper then aluminum but very nice to look at.  Some go with wood look by using cedar panels again nice finish and durable if installed correctly, if you like a natural wood look.   Fiberglass sheets are not to bad and can be painted or wrapped for finishing.   I have even seen one trailered tiny on the road using pine shiplap siding (if installed properly a very cheap sturdy exterior).