The indentation size seems to be hard-coded in PrimsTextLayout: 


    private float getTabAdvance() {
        float spaceAdvance = 0;
        if (spans != null) {
            /* Rich text case - use the first font (for now) */
            for (int i = 0; i < spans.length; i++) {
                TextSpan span = spans[i];
                PGFont font = (PGFont)span.getFont();
                if (font != null) {
                    FontStrike strike = font.getStrike(IDENTITY);
                    spaceAdvance = strike.getCharAdvance(' ');
                    break;
                }
            }
        } else {
            spaceAdvance = strike.getCharAdvance(' ');
        }
        return 8 * spaceAdvance;
    }