1. Small boats (15 feet and under) are generally better off with a small compact downrigger with an arm length of 20 to 24 inches.
2. Large boats (22 feet and up) are generally better off with longer arm downriggers with booms reaching from 30 inches to 48 inches or more.
3. Intermediate size boats (16 to 21 feet) can go shortarm or longarm depending on the type of fishing you are doing, the location where you will mount the downriggers, and the number of downriggers you want to run on your boat.
In the small boat, you are generally sitting down to operate the downrigger. Consequently you want a shortarm unit that you can easily rig by reaching over the side of the boat. If you go with too long an arm you will have difficulty hooking your weights and setting your release. Long arms on small boats can also present a significant safety hazard if the downrigger weight hangs up on the bottom when the boat is being pushed by a strong wind or tidal current. In severe cases a longarm downrigger could tip a boat over in these circumstances.
With larger boats you have a choice of arm length. If you want to fish two or more downriggers on the boat you probably will want 30 to 48 inch arms out each side of the boat and one or more other downriggers straight over the stern. The longer arms out the side minimize the risk of lure tangles during turns. They also provide more fishing coverage. With two 48 inch downriggers out the sides of a boat with an eight foot beam, the downrigger wires will be 16 feet apart. This provides a good fishing spread.
Large boats with high freeboard also need longer arm downriggers to keep weights from banging against the side of the boat. As soon as the lead weight breaks the water surface it begins swinging particularly in rough conditions. A long arm adds a factor of safety.
Another factor with the bigger boat is how you will operate the downrigger. On larger boats you will undoubtedly operate the downrigger standing up. Some downriggers are laid out in a way that makes them very hard to operate from one side or the other or with your left hand. You should think about these factors.
Electric vs. Manual
With respect to an electric downrigger vs. a manual there are two primary considerations. The first is simply convenience. If you can afford it, its nice to be able to push one button and bring up your downrigger. However, don't make the mistake of putting a large electric on a small boat. You should consider the same size and arm length factors as stated earlier. In my mind the real justification for an electric comes if you are consistently fishing deep. By deep I mean more than 100 feet down all day long. In these circumstances an electric will take a great burden out of your fishing particularly when you consider the heavy weights needed to get to these depths.
Where you mount a downrigger on your boat also has a bearing on the downrigger you should buy. If you can mount over the stern of the boat or on the side within a foot or two of the stern a short arm downrigger (24 to 30 inch boom) will usually do. If you have a swim platform you will need a longer boom to clear it. If you mount over the side of the boat more than a few feet forward of the stern you will want to consider a longer boom such as 48 inches. The reason is that as your boat turns a short boom downrigger mounted forward likely will have the wire scraping the side of the boat or worse yet near your prop. In this instance a longer boom will work much better.