There does not seem to be a way to use a custom sized icon in the menu of XFCE. This is what I cobled together from different online resources:
 - Install and enable whisker menu
 - Create a transparent image (see comment for a example file)
 - Choose this transparent image as the icon for whisker menu and select to show icon only
 - create a gtk.css under ~/.config/gtk-3.0/ with the following contents (change size as wanted):
```
#whiskermenu-button {   
  background: @launcher_bg url("/path/to/icon") center;
  background-repeat: no-repeat;
  background-size: 64px 64px;
  margin-top: -2px;
  padding-bottom: 0px;
}
```
 - Finally logout and login again to apply the css setting.