What TheLBLman said, about size limitations and availability. I only order from Amazon because I'm usually ordering in significant quantity (two cards for every camera). Depending upon how often you check cards and what length and resolution video you are collecting, a 64 GB card is fine. I just use 128 GB because 1) my cameras will accept them, and 2) they aren't that much more expensive than a 64 GB, so why not? For my cameras, an HD (1920x1080) video of 10 seconds @ 60 fps is 50 MB in size. A 20 second video 100 MB. A 64 Gb card will hold 1,280 10 second videos, or 640 20 second videos. A 128 GB card, double those numbers.