Downloading is when you pull a picture from the Internet to your computer. The only criteria is a fast connection.
If you mean upload from your computer to the Internet, then small is good. Most websites, like this one, don't need huge pictures because it impacts their bandwidth and storage capacity. Small pictures display just fine.
Digital cameras produce huge pictures that are great for printing, not so great for the Internet. Pictures don't need to be over 400K. 40K is better.
To shrink the image, open the image in Microsoft Paint, select Image, Stretch/Skew, put in 25% for Horizontal and Vertical and hit Ok. Save the file under a different name to preserve the original. Use Windows Explorer to check the file size.
If you have a lot of images you can download a photo managing software like Graphic Workshop and do a batch at a time.