it really depends on the image, if it's simple as the bezel above, the size will be OK, but if it has a lot of details, this will be to small.
remember that the higher the resolution is the better the tracing will be.
oh and one more thing, sometimes, digital photos presents distorsion, because they were taken with a small angle, this could be a problem, and need to be fixed before we can start tracing the art. some guys here are really good in fixing images