Thumbnails
Most thumbnails are 100 pixels in size (on longest side) including a one-pixel black keyline. File sizes are kept to 3.5k or smaller.

Thumbnails used in the panorama galleries are larger, usually 50px high by whatever width the photo dictates.

Photos
Most photos are 600px on the longest side in size and I try to get a good looking image with a file size of around 75k. If I can't do that I err on the side of the photo looking good and the file getting bigger

Panoramic photos can be up to 1000px and 150k in size depending on the image.