You can override all the editor height:
<script>
$.fn.datagrid.defaults.editorHeight=25;
</script>
You also can set the 'height' for the combobox.
field:'status',
type:'combobox',
options:{
height:25,
panelHeight:'auto',
data:[{value:'',text:'All'},{value:'P',text:'P'},{value:'N',text:'N'}],
...
}