Is there a limit to how big a plotscript can be?
Yes, but in practice, no. The limit on individual compiled scripts is currently 256 kilobytes, which would be a script tens of thousands of lines long, a limit almost impossible to reach except for scripts generated by computer program. You should never worry about your script being too big, unless you actually see a Script exceeds maximum size error.
Also, a script can use at most 100 local variables.