Next to each option, you'll see the size (in MB) of the video. Depending on the video's size and length, it could take anywhere from a few seconds to close to a minute to download the video completely. If you're downloading a GIF, you won't be asked to choose a size since there will only be one. Once it's finished, the video or GIF will automatically be saved to Photos, and you can view, edit, and share them from there.