Question: Is it possible to change strings (content and size) in Lua bytecode so that it will still be correct?
It's about translating strings in Lua bytecode. Of course, not every language has the same size for each word...

Answer: 
Yes, it is if you know what you're doing. Strings are prefixed by their size stored as an int. The size and endianness of that int is platform-dependent. But why do you have to edit bytecode? Have you lost the sources?
You can try using the decompiler [LuaDec](http://luadec51.luaforge.net/). The decompiler would allow the strings to be modified in generated Lua code similar to the original source.

[ChunkSpy](http://chunkspy.luaforge.net/) has [A No-Frills Introduction to Lua 5.1 VM Instructions](http://luaforge.net/docman/?group_id=83) that may help you understand the compiled chunk format and make the changes directly to bytecode if necessary.