The max string size is 65535 bytes according to this and other SO posts:
https://learn.microsoft.com/en-us/cpp/c-language/maximum-string-length?view=msvc-160
I have been trying to google working around such as splitting strings into several and then joining them or using define.
If I download the base64 string or read it from a file it works fine even though that still ends up being a std::string = readfromfile etc.
I am sure this topic has been touched on many times but I cannot find a definite answer on workarounds. I am assuming there are ones for such a popular mainstream language to not be held back by a somewhat arbitrary number.