It has been many, many years since I last used a Windows machine, so can't provide any help there! A lot depends upon what you're doing with photography, what you like to shoot and how frequently, and also, yes, the gear that you're using as well. I honestly can't offer any suggestions about using a MBA for image processing and storage since it has also been quite a few years since I last had one and since at least right now at the present time an MBA would not serve my particular photography-related needs.
I shoot a lot (pretty much daily), I have cameras with a lot of megapixels and I shoot in RAW, rather than straight-out-of-camera with .jpg. This then involves culling and then working with editing images in one or more software programs designed for this purpose. I don't use Apple's "Photos" program as it never has and still does not meet my needs.
That said, a lot of people do use Apple's "Photos" program and it works out well for them. Definitely this would be a good starting point if someone has not already been doing much with shooting photos and then editing them. It's a different process from shooting images with an iPhone and then just quickly sharing them with family and friends.
Moving past all that, to answer your question, if I were looking to buy a MacBook Air for use for working with photos as well as for everyday general use, too, I'd go with a maxed-out 15" one. This will give you some decent screen real estate, which is needed when viewing and deciding how to adjust/edit images, and it also will have the maximum amount of storage space (SSD) and RAM, both of which will be valuable to you.
In addition to that, though, I would also recommend purchasing one or more external drives, either HDD or SSD, to use as backups/supplements to the computer. Actually, backups of one's critical and important files are absolutely necessary, whether dealing with photos or not, and if you' re not already backing up your work, then start doing so!