Keep in mind that the solar panels are not for charging your batteries from dead flat to fully charged. These panels do a great job of keeping your battery topped off and ready to go when you are. Most folks on this, and other sailing forms, are weekend warriors, sailing one or two days a week at most. The remainder of the time their boats are tied to the docks. I had the 350-mA charger on my Catalina 27 and it kept too dual purpose batteries fully charged all the time. In fact, this past winter I did not remove the batteries from the boat because the panel kept them fully charged. I checked them weekly with a hydrometer and the SG always read 1.275, which is right where it should be. And, the panel worked just fine on overcast days, times when there was no direct sunlight whatsoever. Granted, it didn't pump out the maximum wattage, but it still put something into the battery, which is better than nothing at all.
My panel was (is) mounted on the back rail on an adjustable frame that allows you to change the position so you get the best benefit from the sun at any time of year. It has a hermetically sealed connector where the wire plugs into the panel, and a pair of battery clips at the other end. About once a month I clean the batter clips with a bit of WD-40 and a piece of cloth. I also wipe down the battery terminals as well with WD 40 to insure good continuity.
Yes, the 15-watt panel is about 2-feet long, which is relatively small when you consider what it does. I suggest mounting the panel somewhere other than on a deck surface. Think about it. If you mount that panel where someone would normally walk, you know someone will likely step on it. If they do, that's the end of the panel. The adjustable, rail mount is the most likely candidate, particularly if it's mounted on a stern quarter.
Having sold my Catalina 27 just a few weeks ago, and the solar panel with it, I just ordered the 15-watt panel for my Morgan 33 O.I.. The Morgan has a pair of starting batteries, plus three golf-cart batteries connected in a series/parallel rig. I'm fairly confident the panel will keep them fully charged.
The boat also has a large, wind generator, but that will only be used while the boat is attended or underway.