Each marker is custom scaled to 15m using Custom SLD styling.

<?xml version="1.0" encoding="UTF-8"?>
<sld:StyledLayerDescriptor xmlns="http://www.opengis.net/sld" xmlns:sld="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc" xmlns:gml="http://www.opengis.net/gml" version="1.0.0">
  <sld:NamedLayer>
    <sld:Name>Custom External Markers</sld:Name>
    <sld:UserStyle>
      <sld:Name>Custom External Markers</sld:Name>
      <sld:FeatureTypeStyle>
        <sld:Name>name</sld:Name>
        <sld:Rule>
          <sld:Title>Bulldozer</sld:Title>
          <ogc:Filter>
            <ogc:PropertyIsEqualTo>
              <ogc:PropertyName>category</ogc:PropertyName>
              <ogc:Literal>bulldozer</ogc:Literal>
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>
          <sld:PointSymbolizer uom="http://www.opengeospatial.org/se/units/metre">
            <sld:Graphic>
              <sld:ExternalGraphic>
                <sld:OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="https://images2.imgbox.com/16/41/WTQ3ZFXN_o.png"/>
                <sld:Format>image/png</sld:Format>
              </sld:ExternalGraphic>
              <sld:Size>15</sld:Size>
            </sld:Graphic>
          </sld:PointSymbolizer>
        </sld:Rule>
        <sld:Rule>
          <sld:Title>Roller</sld:Title>
          <ogc:Filter>
            <ogc:PropertyIsEqualTo>
              <ogc:PropertyName>category</ogc:PropertyName>
              <ogc:Literal>roller</ogc:Literal>
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>
          <sld:PointSymbolizer uom="http://www.opengeospatial.org/se/units/metre">
            <sld:Graphic>
              <sld:ExternalGraphic>
                <sld:OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="https://images2.imgbox.com/68/f8/AVMrkrKz_o.png"/>
                <sld:Format>image/png</sld:Format>
              </sld:ExternalGraphic>
              <sld:Size>15</sld:Size>
            </sld:Graphic>
          </sld:PointSymbolizer>
        </sld:Rule>
        <sld:Rule>
          <sld:Title>Excavator</sld:Title>
          <ogc:Filter>
            <ogc:PropertyIsEqualTo>
              <ogc:PropertyName>category</ogc:PropertyName>
              <ogc:Literal>excavator</ogc:Literal>
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>
          <sld:PointSymbolizer uom="http://www.opengeospatial.org/se/units/metre">
            <sld:Graphic>
              <sld:ExternalGraphic>
                <sld:OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="https://images2.imgbox.com/b1/e5/aCC7uqee_o.png"/>
                <sld:Format>image/png</sld:Format>
              </sld:ExternalGraphic>
              <sld:Size>15</sld:Size>
            </sld:Graphic>
          </sld:PointSymbolizer>
        </sld:Rule>
      </sld:FeatureTypeStyle>
    </sld:UserStyle>
  </sld:NamedLayer>
</sld:StyledLayerDescriptor>