Mitch, it's like geof_junk says, if the area you're detecting is narrow and tight then a small coil may be the best option however if you have deep gravel then you may opt for a large coil that will detect deeper.
As for the detector, it will most likely be the same as you'd use anywhere else. When looking for a gold detector you will usually get the best results with the most expensive one you can afford. People have success with the Gold Monster but it will never compare with a GPX6000 or GPZ7000 although you might chose the smaller detector if you don't get to use it on a regular basis.