Yes, all objects (strings, tables/arrays, ...) need to have a known maximum size. But that's by no means specific to Lua.
There are a lot of implicit assumptions on the type of real time system in my two short comments above - that's why I finally asked for some information on the kind of real time system you have in mind, because maybe all my attempts to explain could be a waste of time for both of us, if you have completely different requirements than I had and you are looking for something completely different.
By "real time" I mean some loop running in some fixed cycle (somewhere between 100 microseconds and 10 milliseconds) or event/interrupt driven. Every cycle starts with reading some input values (booleans and numbers) corresponding to sensor signals. Inputs could also be "messages" (data from serial communication, network packages, ...) - these are of type string but with a well known maximum length (the serial interface has a maximum baud rate, you know the cycle time --> you can calculate it). Within every cycle you need to process all (relevant) inputs, e.g. by filtering it and storing results in a log (data logger), by calculating outputs (closed loop control), probably updating some internal state machines, ... In any case, you must never miss any cycle, because the input data will be gone, the data logs incomplete, the outputs and state machines incorrect. However, the data processed in every cycle has a more or less constant, and limited size. There is no handling of "lengthy" (thousands of bytes) strings.
If you have something completely different in mind, all my suggestions above may not be helpful at all - so probably you should explain a little more on your use case (I don't want to bother you with irrelevant stuff).
