The ImageSize option in Plot should solve this. The simple option values are Tiny, Small, Medium, Large, but you can also specify a specific size using numbers (see the ref page for ImageSize). Here is an example using the simple input:

<msp:evaluate>
MSPManipulate[ Plot[ Cos[var+x], {x,0,2Pi}, Frame -> frame, ImageSize->Small], {var, 0,20}, {frame, {True,False}}, OutputSize->{621, 384}]
</msp:evaluate>

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>