Coming from a photography background, I always got 77mm filters since they are the "standard" for "professional" lenses. Then I would just get step-up rings like a 62 to 77 for my Sigma 30mm 1.4, 52 to 77 for my Canon 50mm 1.8, etc.
You could stack filters but that's adding more glass in front of your lens. This increases the chance of unwanted flares, glares, etc. but also can degrade image quality. When I don't need a ND filter, I sometimes shoot with no filter at all.
One example was when I shot a video at night, obviously a ND filter wouldn't help. A UV filter on my Sigma 30mm 1.4 created glares that were annoying.. like they'd move when the camera moved. Don't use filters if you can, and use them if you have to.