To custom the combogrid font size, please define a special CSS class.
<style>
.textbox .mc,
.mc .datagrid-cell,
.mc .datagrid-cell span{
font-size: 14px;
}
</style>
And then apply the '.mc' class to the combogrid.
var cc = $('#Company');
cc.combogrid('textbox').addClass('mc');
cc.combogrid('grid').datagrid('getPanel').addClass('mc');