JEditorPane and JTextPane

JEditorPane is the right component to display HTML. A HTML file is streamed into the editor pane and therefore the pane doesn't know its true preferred size until the HTML file was entirely read.
JTextPane displays a StyledDocument which, for similar reasons, doesn't have a (trustworthy) preferred size.

As a consequence, you must add JEditorPane and JTextPane to a JScrollPane and set the scroll pane's preferredSize property.
Don't be fooled by the fact, that a JEditorPane or JTextPane you insert into a design grid, is (most often) displayed like it should - RADi calculates the layout's preferred size multiple times.

See also:
JScrollPane
The design grid
Components in grid cells