I have some small photographs (3½"x3½") to scan and enlarge slightly. What is the optimum resolution to use?

You're better off scanning at a higher resolution than resampling later, so scan at the final size and resolution you want (usually 300ppi is plenty). If you can't set the size, err on the "too-much" side and scan at 600ppi or higher. Then resize down in Photoshop.

Thank you Doug