Have you tried the OutputSize option to MSPManipulate? The webM example webMathematica/BrowseExamples/Manipulate/SliderPlot.html shows this
<msp:evaluate>
MSPManipulate[ Plot[ Cos[var+x], {x,0,2Pi}, Frame -> frame], {var, 0,20}, {frame, {True,False}}, OutputSize->{621, 384}]
</msp:evaluate>