I hate it when someone says that some political unit would be the "13th largest" or "27th largest" country if independent - without defining what they mean by "large."
In the case at hand, I suspect that he means "largest economy," not "largest population."
With regards to population, I believe that Texas would be about 45th or 46th (similar to Ghana or Nepal).
With regards to geographical extent, I believe that it would be about 39th or 40th (similiar in geographical extent to Sambia or Burma).
Regards,
Texas is 1.1 times the size of France.