“I have both the Sony a7Rii and a6500 mirrorless cameras and was unsure of which would be the correct size PRO Camera Rain Cover so I purchased both the 12" and 16" versions. I like the 16" version better as it gives me more room to place my hands inside the cover to access both a flash and the camera's controls. I decided, however, to also keep the 12" version to use with an older 4/3 Panasonic that I use for infrared images.
Please note that Bob "upgraded" me at no cost to the new PRO II versions that include neoprene tabs so that the cover will grip the lens/hood better. The tabs appear to prevent the cover from slipping when the cord is cinched so it seems to be a good upgrade.