A couple of points:
1 All advice seems to be US related. If you are in a different country make sure that what is recommended is legal locally (For example I am not sure that you can get away with not refunding shipping in the UK)
2. If there is a huge size range then it is perfectly reasonable to have multiple shipping rates. The firm that I have bought garden equipment from has three flat rates, indicated in the catalogue by an icon beside each item showing either an envelope, a parcel or a truck.