Export["file", Manipulate[...], "VideoFrames"] exports an animated demonstration of a Manipulate object to a series of raster image files, using the specified file name for constructing a numbered sequence of file names.
All exported images will have the same dimensions.
Export exports a list of images, graphics, or arbitrary expressions, converting each element to an image file.
Export infers the image format from the file extension in Export.
With a setting ImageSize->Automatic, the first frame of the exported object will be rasterized and taken as the size of the resulting image file. Subsequent frames will be padded to match the dimensions of the first frame.
All options specific to the exported image format can be given.
Additional options are available when exporting Manipulate, Animate, and ListAnimate expressions. See the documentation pages for "SWF" for full information about supported options.