To set an upper limit, simply add an upper attribute to the populationSizes parameter in the generalizedSkylineLikelihood element:
<generalizedSkyLineLikelihood id="skyline"> <populationSizes> <parameter id="skyline.popSize" dimension="10" value="100.0" lower="0.0" upper="1e7"/> </populationSizes> <groupSizes> <parameter id="skyline.groupSize" dimension="10" value="1.0" lower="1.0"/> </groupSizes> <populationTree> <treeModel idref="treeModel"/> </populationTree> </generalizedSkyLineLikelihood>