Both of your queries can be achieved with SimpleViewer-Pro but not with SimpleViewer-Standard (the free version).
1. change the size of thumbnails - so instead of regular squares, it will be maybe wider?
You can define the dimensions of the thumbnails using the thumbWidth and thumbHeight Pro Thumbnails options. Please bear in mind that the thumbnail images fetched from Flickr will be the small 75 x 75 images so if your thumbnail dimensions are larger than this, they will be scaled up and may lose quality.
2. change the opacity/color of thumbnails? so for example all thumbs will be grayed out, or b/w, except for the image shown on the big screen.
You can dim all thumbnails except the currently selected one by setting the Pro option imagePreloading="NONE" (though this will also prevent images from being preloaded). The brightness of the thumbnails can be set via the thumbLoadBrightness Pro option.
SimpleViewer Support Team.