Place the following code in Custom HTML in the form where it contains the form grid / list grid.

Form Grid

<script>
    $(function(){
        //make formgrid popup wider
        FormUtil.getField("items").find("#width").val("90%");
        FormUtil.getField("items").find("#height").val("90%");
    });
</script>

List Grid

<script>
    $(function(){
        //make listgrid popup wider
        FormUtil.getField("items").data("width", "90%");
        FormUtil.getField("items").data("height", "90%");
    });
</script>

Replace "items" with the field ID of the grid element appropriately.