If editing dozens of Box plots is no fun for you,
you can access the FrameBox via JSL scripting -  and then talk to the Marker and BoxPlotSegs: 
 
fb1 = current Report()[Framebox(1)];
markerseg = fb1 << find seg("MarkerSeg");
markerseg << set marker size(10);
// talks to ALL Box plot:
boxSegs = fb1 << find Segs("BoxPlotSeg");
boxSegs << set line width (3)