Standard shock bushings are usually 1/2" inner diameter (or 12mm on some older Rockshox shocks). If you use a bushing that has an offset hole, then its possible to lengthen or shorten the shock eye-to-eye. Generally, if you're shock uses 6mm diameter bolts then each offset bushing can give you a +/- 3mm change in eye-to-eye length. If you use two offset bushings (one at either end) you can get a maximum change of +/- 6mm in length.
If your shock uses 8mm mounting bolts then the offset possible is less (typically between 1.5 and 2mm). Also, on some bikes (like the Giant Anthem) there is no shock bushing used at one of the shock, so the only one offset bushing can be used.