There was an issue before with sizes over 64kB. That may have been adjusted in recent versions. I haven't looked over the changelog. However, if you plan to use the ini file outside of mIRC, you may run into issues depending on the program. I don't think mIRC was the only one with a limit.
Also, in general, INI files shouldn't be that large. If you have one that large, then there is probably a more efficient method of storing the information. Probably hash tables. It may be worth looking into.