Hello,
Thank you for your question. I have been looking into your question and we do not provide a built-in approach to increase the size of the filter icon. You can always change the icon by re-templating. I am attaching a sample application(xamGridFilteringIcon.zip) that shows that suggestion.
You can find the default templates for XamGrid in :
C:\Program Files (x86)\Infragistics\NetAdvantage 2012.2\Silverlight\DefaultStyles\XamGrid.generic.xaml
If you need any further assistance on this matter, feel free to ask.