The Secure Digital card--SD card for short--possesses an incredible amount of storage space for its size. What you can store on it, however, depends heavily on several factors.
Storage on Card
Video media comes in a wide variety of forms these days--MP4, MPEG, and FLV are just a few of the types out there. Each format has its own compression, quality and average file size characteristics. The file size is what determines how much video you can fit on one card. As an example: You have a 2GB SD card, and four files of roughly 600MB--approximately 2.4GB. Your card will not hold all four files, but it will hold three of them quite handily. You can expect to fit one hour of average quality video per gigabyte on a card; your mileage will vary based on the format and quality of your media.
Is Bigger Better?
In short, yes. The larger your SD card's storage space is, the more video you can fit on it. SD cards cover a wide range of storage capacities, from under a gigabyte to over 60GB. There are even plans to increase that maximum size up to two terabytes--that's more than 2,000 gigabytes, for the curious, or well over 500 hours of DVD quality video. However, you should pay attention to the read/write speed of the card you're purchasing. Faster is also better in most respects, but the average user won't need the absolute best.
Types of SD cards
The original SD card, with its limited storage, is fast being outmoded by newer standards with much greater capacities. Currently on the shelf is the SDHC variant, with storage ranging from 2GB to 32GB. Newest to the market is the SDXC, which is projected to hold amounts of data into the terabytes. When shopping for a card, you should bear in mind which SD types your devices can handle. This information will usually be listed in the manual or somewhere on the device itself.