As JEditorPane is designed to support different kinds of content types, it does provide a way to specify a "tab size" directly, because the meaning of that should be defined by the content model.
However when you use a model that's a PlainDocument or one of its descendants, there is a "tabSizeAttribute" that provides what you are looking for.
Example:
JEditorPane pane = new JEditorPane(...);
...
Document doc = pane.getDocument();
if (doc instanceof PlainDocument) {
doc.putProperty(PlainDocument.tabSizeAttribute, 8);
}
...
From the Javadoc:
/**
* Name of the attribute that specifies the tab
* size for tabs contained in the content. The
* type for the value is Integer.
*/
public static final String tabSizeAttribute = "tabSize";