Sam,
Charging a 50% depleted bank of 200 amp-hours vs charging a 25% depleted bank of 400 amp-hours takes exactly the same solar panel wattage.
If the bank is not large then more solar wattage is a good idea because you need to get back to 100% state of charge. So small amp-hour banks require a higher ratio of wattage to do that. The maximum sensible number is about 150 watts per 100 amp-hours, because the acceptance rate on for a flooded lead acid jar is about 12.5 amps at 85% state of charge.
What is important is to not drop below 60 watts per 100 amp-hours of storage. That way flooded batteries can be equalized on solar. On a 400 amp-hour bank that is 240 watts--but on a 200 amp-hour bank you may want to do 300 watts.
When I was not full time my 256 watts of solar serviced 875 amp-hours of battery bank. I did use switches to allow equalization to take place.
