Ian,
In the templates folder open the Print.html and make changes to the width, height and dpi inputs:
i.e. data-dojo-props="name:'dpi',required:true,value:96,style:'width:100px;'"
<div data-dojo-type="dijit/form/Form" data-dojo-attach-point="mapOnlyFormDijit">
<table cellspacing="5">
<tr>
<td>
${nls.width}:
</td>
<td>
<input type="text" data-dojo-type="dijit/form/NumberTextBox" data-dojo-props="name:'width',required:true,value:670,style:'width:100px;'"
/>
</td>
</tr>
<tr>
<td>
${nls.height}:
</td>
<td>
<input type="text" data-dojo-type="dijit/form/NumberTextBox" data-dojo-props="name:'height',required:true,value:500,style:'width:100px;'"
/>
</td>
</tr>
</table>
</div>
<div style="font-weight:bold;">
${nls.printQualityOptions}:
</div>
<div data-dojo-type="dijit/form/Form" data-dojo-attach-point="mapQualityFormDijit">
<table cellspacing="5">
<tr>
<td>
${nls.dpi}:
</td>
<td>
<input type="text" data-dojo-type="dijit/form/NumberTextBox" data-dojo-props="name:'dpi',required:true,value:200,style:'width:100px;'"
/>
</td>
</tr>
</table>
</div>