Another vote for the Halfords medium size here. It does what it says on the tin. Not sure how bad it is for the paintwork as I've used it on my silver coupe which doesn't have the best paint to start with.
If I was going to leave on long term I'd do what Gman suggests and use a soft one underneath.
I did notice the other day that mine is slightly red from the door mirrors are on the car, but the lacquer came off them many years ago and they don't look to have suffered particularly in terms of the amount of paint now on them. I'll polish them before autumn when the cover goes back on though.