There are many cover manufactures out there. From custom made (very expensive) to off the shelf. All covers only last a few years.
I have had ADCO in the past and more recently a cover from Nationalrvcovers.com. The one from National Covers seems to be made better and more durable (so far).
One caution is that covers tend to slap the side of the RV in the wind and can wear against the the RV. Especially at the corner between the roof and the side. They can wear on your rubber roof. I protect mine with custom made foam pieces.
Many will say that covers are not necessary. They probably aren't really. But they do, in my opinion, keep the RV cleaner during winter storage.
Manufacturer web sites will give you directions on how to measure. Most just have you select the type of RV and then measure the full length form end to end including ladders and bumpers.
Be very careful when putting a cover on or off. It's easy to trip and fall off the roof.