XML/XML Schema/length

xs:length defines a fixed length measured in number of characters or bytes (xs:hexBinary and xs:base64Binary)

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">

  <xs:simpleType name="standardNotations">
    <xs:restriction base="xs:NOTATION">
      <xs:length value="8" />
    </xs:restriction>
  </xs:simpleType>
</xs:schema>