Simple test of the minscriptsize attribute on the mstyle node.
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mfrac>
<mn>2</mn>
<mi>x</mi>
</mfrac>
<mo>=</mo>
<mstyle scriptminsize="20pt">
<mfrac>
<mn>2</mn>
<mi>x</mi>
</mfrac>
</mstyle>
</math>
Author: Design Science, Inc (D. Doyle)