For a combo box/drop-down list, you are correct that this is currently a limitation of DashBoard's UI widgets. Font size changes are applied to list widgets (non-drop-down), buttons, etc. We can put combo boxes in the future.
In the meantime, have you tried putting DashBoard into "Large Font" mode to see if this is sufficient? Window -> Preferences -> Genera -> Use large fonts where possible?
For touch-friendly UIs that we create, we typically will use "Toggle Buttons" as the widget type.
If you still want to hide/show the choices, there is a bit of a workaround available using the tag:
<abs contexttype="opengear">
<meta>
<params>
<param access="1" constrainttype="INT_CHOICE" name="a" oid="a" precision="0" type="INT32" value="0" widget="combo">
<constraint key="0">A</constraint>
<constraint key="1">B</constraint>
<constraint key="2">C</constraint>
<constraint key="3">D</constraint>
<constraint key="4">E</constraint>
<constraint key="5">F</constraint>
<constraint key="6">G</constraint>
</param>
</params>
</meta>
<table height="78" left="55" top="29" width="336">
<tr>
<param colspan="1" expand="true" fill="both" oid="a" onmousedown="ogscript.reveal('selector');" rowspan="1" style="bdr:line;bg#listbg;fg#listfg;size:Bigger;" weightx="1.0" weighty="1.0" widget="label"/>
<popup colspan="1" fill="both" id="selector" name="Select..." rowspan="1" weighty="1.0">
<simplegrid cols="1" height="500" width="200">
<param expand="true" height="112" left="652" oid="a" onmouseup="ogscript.hide('selector');" style="style:toggleButton" top="178" widget="22" width="353"/>
</simplegrid>
</popup>
</tr>
</table>
</abs>
#DashBoard