Do you need a script file that big?!
In reply to your question, I don't think there is a limit in later versions of mIRC... although in the help file, /writeini has a switch:
If the -n switch is specified, mIRC will attempt to write to the .ini file even if it is larger than 64k.
Prehaps there's a limit of 64 for script files also? don't know!