I have used ASPJpeg with great success. It allows you to have a lot of control over the size and to also sharpen the images. I am sure there are cheaper solutions but I already had ASPJpeg so this was a great option for me.

Also if you have Web Connect 4.2.0 there are two new fuctions, here is the link http://www.west-wind.com/webconnection/docs/index.htm?page=_S8104DGGL.htm. Below is some info from the help file.

Thumbnail image and Image Info functionality
You can now use the CreateThumbnail and GetImageInfo functions in wwAPI (note they're functions not methods) to create scaled thumbnail images from larger images while GetImageInfo returns the width, height and resolution of an image. These functions use GDI+ functionality in wwIPStuff.dll. The Photoalbum sample application has been updated to work with thumnails and image info. Note that if you have the photoalbum installed you will have to Reload images on existing image directories to create the thumbnails.