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)