Image Optimizer is really neat service (with a free downloadable version, that can do bulk conversions). I have previously posted about windows live & picassa’s ability to export & email smaller version of photos.
Here is a useful table for common sizes, you can click on the size descriptor to see an example of that image size.
|Suggested Pixels||Max Width||Max Height|
|Small Web Photo|
|Normal Print |
10 by 15cm (4 by 6”)