Spraydeck sizes are based on both the kayak cockpit size and your own waist measurement. Manufacturers commonly call the deck sizes "Keyhole, Bigdeck" etc, but what these mean can vary by manufacturer so make sure you measure the actual sizes rather than just going by the size name.
Kayak Cockpit Covers
Neoprene kayak cockpit covers help keep rain, debris and insects out of your boat during periods of storage. The covers are also of use when travelling with your kayak on your roof.
They first help to stop a swarm of flying insects being scooped up into your boat as you drive along with an open cockpit and also help reduce the drag caused by the open hole. This reduce the strain on your roofrack and also slightly helps fuel efficiency.