How to allow html tags and restrict number of characters‏

Hello All,
 
I am in the process of developing an XSD.
 
I have requirement, there is a tag which should allow html tags in it, it is a description field, users can enter any text with bold, italics, paragraphs etc.
 
And the filed length should be 50,000 characters.
 
How do I achieve this in xsd?
 
Any suggestions are much appreciated.
 
So far I managed to allow html tags but not able to restrict the number of characters.
 
Please find part of my xsd

<xsd:complexType name="htmltagsok">
<xsd:sequence>
<xsd:element xmlns:q1="http://www.w3.org/1999/xhtml" ref="q1:body" minOccurs="0" />
</xsd:sequence>
</xsd:complexType>
 
 
<xsd:element name="Conclusion" type="htmltagsok" />

Regards,