The Snapshot Options window provides options that determine the size of the snapshot produced.
Snapshot Size
A number of snapshot sizes ranging from 160x120 to 1600x1200 are available.
If you would like to produce an anti-aliased image, you can save your animation still-frame image larger than the desired result and use image editing software to scale the image to the appropriate size. Most image editing software will scale using interpolation, which results in super-sampled anti-aliasing.
Note that you will not be able to select a snapshot size larger than your current screen resolution.