More often the problem is that you need one longer than can be found. Most are only 20' long, which is a problem if you have a 20' boat. I hunted for a long time to find a company that made 25' straps so I could load my 18' boat, I often need the strap to reach several feet beyond the trailer where I can hook the the boat up. I then take up the slack and winch it up the rollers.
If the strap was exact length, I'd have to get wet.
I have a 14' jon boat and have used all 20' of the winch strap before, sure glad it wasn't shorter. As you use the strap it will tighten and get lower on the spool, plus, I'd rather have a full winch, more length of strap pulled for every turn of the handle.
come in handy later on, when you don't replace the strap, it breaks, you still have enough strap to get the boat on the trailer. i also have had to dryload on some very bad ramps. also the more on the drum the faster the winch pulls.
Depending on how much strain is on the strap, that can work both for and against you. If your winching on a heavy boat, the ratio gets worse as the winching gets harder. Of course in that case, the right answer would be a bigger winch and better loading method.