How to set the splitpane's pane to a fixed height value?

Question

I follow the splitpanes docs:

<splitpanes class="default-theme" horizontal style="height: 400px">
  <pane size="65">
    <span>1</span>
  </pane>
  <pane size="10">
    <span>2</span>
  </pane>
  <pane size="25">
    <span>3</span>
  </pane>
</splitpanes>

when I change the screen the panes height will changes too, how could I set the pane width or height to a fixed value?

I tried add px unit, but it do not work.

  <pane size="25px">
    <span>3</span>
  </pane>