The sailboat winch, which is used to wind and unwind the line attached to the sail or anchor, is sized according to how large the boat is, the number and strength of the operators, and its specific use. Boats used for leisure sailing normally require smaller winches that do not need as much physical strength to operate, while racing boats usually have to manage more power and need larger winches, although they can get by on smaller ones, depending on the size of the crew. The size also determines the power ratio.
Measure boat length and sail area. The manufacturers will ask for the exact length of your boat to determine what size winch you need, but they may also want the area of your largest sail -- the genoa -- or the smallest -- the jib. The primary operators will need to be able to handle the winch under the maximum wind load on each sail. The wind load may be higher on the smaller sail because it is used during stronger winds. Once you have a number, double it and choose a winch based on that.
Choose the placement of your winch. The straighter the rope that leads from the winch, the less force you will need, so choose a place with as few angles as possible. If it's necessary to angle the rope leading to the mast or anchor, install a turning block so that the rope leads directly into the winch, not above or below it. If you can work out the proper placement, you may be able to avoid a winch that's too large for your boat and the crew.
Consider an electric winch. Manual winches are traditional for raising and lowering sails, but if you want to sacrifice size for power, electric winches make the job a lot easier. Leisure boaters, retired couples or boaters with physical limitation can benefit from electric winches. Because electric winches tend to be smaller, installation is less of a problem. You still need to size it, depending on the main use and the capacity of power that your boat can spare -- you should have from 50 to 140 amps available. An electric winch should also double as a manual winch.
Calculate a budget. It's possible that you can get by with one winch if you're not a serious sailor and strong enough to handle a manual crank yourself. You can also choose a smaller model. If you need to go for a larger or motorised version, be prepared to pay more. Also, stainless steel is more expensive (and more durable) than aluminium or bronze.
To calculate the power ratio, multiply two times the handle length times the gear ratio and divide it by the diameter of the drum. Single-speed winches don't require the 360-degree rotation of a handle, but operate on a push/pull, back-and-forth motion. They are less tiring to operate. By placing it properly, you may be able to use one winch for multiple purposes.