I'm a bit puzzled by this problem: you haven't shown enough information. Is there a schema? If so, how is the @size attribute defined? I wouldn't expect to see this error if there is no schema (the xs:decimal must come from somewhere!). If there is a schema then I wouldn't expect your source document to pass validation; and I wouldn't expect it to do anything on Saxon-B as distinct from Saxon-SA.
Please note that type-checking is very different between XSLT 2.0 processors and XSLT 1.0, although a 2.0 processor should fall back to behaving in a very similar way to a 1.0 processor when the data is untyped and unvalidated.
Please supply more information about your source document, stylesheet, and schema.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference