There are no any fixed limitation on log size, while if you need to analyze activity for very busy sites (with more than approximately one million hits per hour during peak hours) you may need to change the visitor session timeout value or enable the adaptive timeout in the program options to reduce memory usage.
It's not possible to add new log formats to the program, you can find information on the supported formats here.
It's not possible to add new log formats to the program, you can find information on the supported formats here.