32KB, is a good ratio for small files. 64KB for 640x480 JPEG @ 85% (that should please most shutterbugs). If your site is about photography and imaging, 2MB, as they'll be uploading 5+mpx images.
Keep in mind, as stated above, that folks tend to not check their files sizes and may even want to upload in .raw format (HA!). Unless you have an unmetered bandwidth and plenty of memory for Apache to serve images, you will have to set some file size limit for attachments.
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