A 50 watt panel would put about 30 amp hours into the battery bank over a sunny day. That is many times the self discharge rate. If all you want to do is maintain charge in a 200 amp-hour capacity battery bank a 10 watt panel, which outputs about 1.25 amp hours per hour. Even assuming only 25% efficiency, you will put about 6 amp-hours into the battery bank per day, which is still significantly more than the self-discharge rate.

A 10 amp panel can get away with 16 or even 18 gauge wire unless the wire run is long