[Toby]

You have to analyze that question from a reverse direction. Your thumb size relative to the thumb size of the sites you trade traffic with will have a definite effect on the productivity of those trades.

If you're trading with a site with thumbs substantially larger than yours, your traffic to them will most likely be very good so long as their content is a good match for yours. Conversely their traffic back to you will most likely have a fairly poor productivity. This disparity makes it difficult to maintain an equitable trade.

What you need to do is look at other sites you're likely to trade traffic with, get a general idea of the consensus on thumb size and pick one somewhere in the middle that still works with the site layout you have.

As for vertical vs horizontal, that's pretty much a pic vs movie issue. Having both can be a bear to work into a page layout, so it depends more on what type of gallery will be more common on the site. An orientation that matches the majority of the site content will usually result in thumbs that best show off the goods.

If I didn't have databases full of 120x160 thumbs I'd have to resize I'd probably go with 135x180 if building a new TGP today. I've been using that for building my own galleries for over a year now.

P.S. I hate square thumbs. It's a compromise to try to cover both vertical and horizontal, and ends up not doing either justice.