Size is a big issue when it comes to covering things like a bike or a car. You will find that most car covers are one size fits all or a little bit bigger then you need it to be. If you get something with the drawstrings you will be able to give the car or the bike a customer, snug fit that you need from a good cover.
You will find that it is very important that you see the benefits of having that custom fit, but also you will find that it will allow you to keep the material a little loose. Mold is a big factor in fit. If it is too snug then you will have an issue with mold and moisture and no one wants to deal with that!