There is no documented limit for the maximum file size or R code files. In fact, R will be able to deal with anything that’s even remotely reasonable. But for what it’s worth a 5 MiB source code file is not reasonable. If you actually have such files, I strongly suggest removing the large data that’s declared inside it, and moving it to a proper data file instead: separate your code and data. Actual code will never be this big.
As for editing such a file, different code editors have different limits for the size of files they deal well with. Again, having such a big code file is plain unreasonable, so not many code editors bother catering to this use-case, and even though few editors have a hard-coded limit, interactively editing such large files may not work.