It really depends on your goal and preferences, but there are a few things to consider. I aim for 500x500 for image size, but I am slightly protective of my photos and don't like to put it all out there. Here's my post explaining why I don't provide larger images on my blog and perhaps why you should consider using smaller images for Instructables. The automatic resizing isn't very attractive on Instructables, and smaller images look better in thumbnail and in the steps. I'll admit 500x500 is on the small side. You could go to 800x800 for readers who would like to see more detail. Very large images just aren't very viewable on most computer screens. I don't like to go above 1024x768 unless I'm providing a wallpaper. If more detail is needed, I just provide the pertinent portion of the original.