Try to make the big image at least 600pix wide on the short side. And for the thumbnails I'd say at least 120 on the short side would be good (~21600 pix²). I've also used thumbnails with a size up to 300x200 for TGP galleries, it all depends how it suits your layout.
Don't make them too small, cause content is IMHO the biggest selling factor on your gallery, so make sure that the surfer actually sees the quality of your content right when he loads the gallery
