Are you talking about the apache log file sizes in apache/domlogs? If so, by default, these are processed every 24 hours for each domain and cleared out. If they are getting that large it is because the site is getting a lot of traffic or there is a problem with log processing.
There is no provision to limit there size per se, but you can set it to process the logs more frequesntly which would cause them to clear out more often and thereby limit their size. In WHM to got statistics configuration and in the lower right, there is the schedule area and set it to process however often you see fit.