Photoblog - How big should the photos be?

For the sake of your viewers, you should resize to a web-friendly size. If you upload the full size but limit the display size, the full sized photo still has to be downloaded and this will make the page perceptibly slower. Probably for a blog page, even 1024 x 768 will seem too large (for many people, that would be full screen). I would try 800 x 600. If you are using Flickr as you storage space, Flickr does the resizing for you and you can upload the full size and then link to the appropriate size in your blog. Then if you are going to let people use some of your photos as desktop images, you can allow them to download an appropriate size from Flickr.