The after-image area differs from all other areas because each extent can be either fixed or variable length. Each extent is treated as its own area. It is fairly common for people to define several (more than 10) variable-length extents for the after-imaging.
To choose a size, you must know how much activity occurs per day and how often you intend to switch after-image extents. You can define all of your extents as variable length and see how large they grow while running your application between switches. To accommodate above normal activity, you need extra extents. If you are concerned about performance, you should have the after-image extents fixed length so you are always writing to preformatted space. Preformatting allows you to gain:
Performance by eliminating the formatting of blocks during the session
Use of a contiguous portion of the disk
Most operating systems are fairly good at eliminating disk fragmentation. However, if you have several files actively extending on the same file system, there is a high risk of fragmentation.
Performance by eliminating the formatting of blocks during the session