If geoserver polygons are small it's nice to have a centroid point that is really big when zoomed out. Remember painter's rendering order.

centroid_when_zoomed.xml

<Rule>
  <MinScaleDenominator>1500000</MinScaleDenominator>
  <PointSymbolizer>
    <Geometry>
      <ogc:Function name="centroid">
        <ogc:PropertyName>geom</ogc:PropertyName>
      </ogc:Function>
    </Geometry>
    <Graphic>
      <Mark>
        <WellKnownName>circle</WellKnownName>
        <Fill>
          <CssParameter name="fill">#F5FF3B</CssParameter>
        </Fill>
      </Mark>
      <Size>18</Size>
    </Graphic>
  </PointSymbolizer>
</Rule>