Hi there
There is no rule of thumb, but if you're lucky you'll have a depth of field preview button on your camera which shows you what dof the chosen aperture will give you.
There is no general rule for the reason that dof also depends on focal length - longer lengths give you less dof. What I would do is stick to the largest 2 or 3 aperture stops on the lens.
The best way is to go to dof master (the site that Gidders linked) to work it out.
The sunny f/16 rule is something completely different - it's a rule of thumb for calculating exposure when you don't have a light meter. Your DSLR doubtlessly has a light meter so you can leave that well alone. With outdoor portraits in bright light requiring a shallow DoF and hence wide aperture you'll have to reduce the shutter speed or use an ND filter which will reduce the amount of light entering the camera.
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