Admittedly, there are two schools of thought: to cover or not to cover. I prefer to cover. I would suggest a custom fit cover. There's less chance of the cover rubbing against the car due to wind, rain or snow.
Here's some links to fabric charts and guides.
http://www.supercarcovers.com/car-cover-fabric-chart.php
http://www.supercarcovers.com/covercraft/car-cover-fabric-guide.php
http://www.supercarcovers.com/coverking/car-cover-fabric-guide.php
Here's two other sites. I went with Covercraft covers for both the '04 Miata and the '02 Impreza. I believe I got them from Big Sky.
http://www.bigskycarcovers.com/our-products/covercraft-car-covers/
http://www.covercraft.com/us/en/shop/1990-mazda-miata/vehicle-covers/custom-fit-carcovers/