The FKM plugin has been designed to be low impact and high performance. By default the Plugin watches for new lines as they appear, rather than the scanning the whole file. Therefore, the file size growth rate is more important than the actual total size.
For files that potentially grow at a high rate, either continually or at specific times of day, there are parameters in FKM Plugin that may help protect the underlying system from any performance impact: maxSamplesLagging, maxLinesProcessed, excessiveUpdatesTimeout. You should refer to the documentation linked for more details.
We recommend that you always use the latest document version in https://docs.itrsgroup.com/ to keep you up-to-date.