There's a lot of variables to consider.... here are a few off the top of my head...
If bandwidth is a concern then you want to keep them small of course. I like a width of 110 for portrait and 130 for landscape. There's also the school of thought that larger thumbnails actually save bandwidth cuz surfers don't need to click for the big pic to see if they like the girl. I have tried both ways and prefer the smaller thumbs. The bulk of surfers are on slow connections so they prefer a fast loading page.
If getting listed on large TGP's is a concern then you really need to find that happy medium between file size, quality, and physical size. Again I find the 3's I listed above to be ideal for me.
As for promotion... I don't think it makes much of a difference. You won't see more sales from bigger thumbs or vice versa... and esthetics really depends on your layout as a whole. If you are using banners then I would suggest a size that allows a flow from the banner to the thumbs. Of course if you make your own graphics you have more freedom.
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Make $$$ the Diabolical Way