create a filter script for example to reduce any model to 32,000 faces script:
<!DOCTYPE FilterScript>
<FilterScript>
<filter name="Quadric Edge Collapse Decimation">
<Param type="RichInt" value="32000" name="TargetFaceNum"/>
<Param type="RichFloat" value="0" name="TargetPerc"/>
<Param type="RichFloat" value="1" name="QualityThr"/>
<Param type="RichBool" value="true" name="PreserveBoundary"/>
<Param type="RichFloat" value="1" name="BoundaryWeight"/>
<Param type="RichBool" value="true" name="PreserveNormal"/>
<Param type="RichBool" value="false" name="PreserveTopology"/>
<Param type="RichBool" value="true" name="OptimalPlacement"/>
<Param type="RichBool" value="true" name="PlanarQuadric"/>
<Param type="RichBool" value="false" name="QualityWeight"/>
<Param type="RichBool" value="true" name="AutoClean"/>
<Param type="RichBool" value="false" name="Selected"/>
</filter>
</FilterScript>
save it as 32k.mlx.
meshlabserver -i original.obj -o new_small.obj -s 32k.mlx