Cache file size and detail is controlled by a combination of the "GI Relative Detail" setting and the main render Detail setting (Sample Quality has an effect too, but does not tend to significantly increase the cache file size). As Richard said, you can use a GI cache file generated at a resolution other than what you're rendering at (though doing so with a different aspect ratio is not advised). The effect of using a lower resolution cache file is similar to just reducing Relative Detail.
Generally the best way to get flicker-free results, at any detail level, is to use a "sparse cache", as Richard also mentions, e.g. rendering a cache file for every 5th (or 10th, in his case, although this would probably be too big a jump for faster moving scenes), and then blending between 3-5 cache files. This is better than a cache file on every frame in general as it creates more averaged results, but tends to maintain the level of detail of your base GI settings still. So essentially, adjust Relative Detail *or* use a lower resolution cache file to control detail vs. render time and quality, whereas use sparse cache and adjust frame blend range (more frames blended = smoother result) to get less flicker.
- Oshyan
