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Some time ago, Script Editor used the Mac's Text Edit toolbox routine which had a limitation of 32k (Script Editor's limit was 30,000 characters). SimpleText, HyperCard and some other programs had the same limitation; and the only editors able to compile larger quantities of code were Smile and Script Debugger.
Actually, it doesn't exist such limit, and you can compile as many code as you wish, as far as the compiler allows. You can't compile literal strings greater than 32,000 characters and seems that lots of lots of handlers will break the compiler sooner than regular code (eg, 202.5 Kb of code consisting on "set x to 5 * 5").
The recomended technique to avoid problems with larger scripts is breaking them into several modules, and use the load script command to "join" them at run time.