A Suzuki dealer can tell you which size you need.
From reading the posts on how to adjust it, it sounds like a bit of a pain. If you have a couple of buddies that can help you adjust it, you can get the sag close. That way you won't be going back to adjust it as much until you get it dialed in. I have only owned one cruiser in my life and it had twin shocks on the outside and so easy to adjust. Your sag is usually set to about 30% of the shocks travel.
Get a couple of buddies to lift the back of the bike up (Or use a jack if you can. Just make sure if you do that it is not hindering the travel of the swing arm.) and measure from a set point on the bike down to a point on the swing arm. Then, lower the bike, sit on it in the position that you would ride in as a buddy holds the bike upright. Then your other buddy will measure the points again. To increase the sag hight, compress the spring, to lower it, decompress it. After that, you can fine tune it to fit your ridding style.
Setting your sag properly, will make the bike handle better as it will increase the rear ride hight and help the bike to turn better. If you are going to be ridding two-up a lot, since your shock is hard to get to, you may want to set it a little more to the stiff side (less sag).
PS, I looked around online and was not able to find the rear travel for it. Any one know?