Thanks Eric.
I already checked out the document but it seems that example only customize the existing font sizes i.e 8, 10, 12......
I have changed the common.xml as below but when I formatting any text with size 9 it actually formatting as 10pt in HTML.
Please let me know if I am missing something here.
<item text="1 (8pt)" value="1">
<html><![CDATA[8pt]]></html>
</item>
<item text="2 (9pt)" value="2">
<html><![CDATA[9pt]]></html>
</item>
<item text="3 (10pt)" value="3">
<html><![CDATA[10pt]]></html>
</item>
<item text="4 (11pt)" value="4">
<html><![CDATA[11pt]]></html>
</item>