The database field where posts are stored has a size limit of 12000 characters. Whitespace and all formatting information also counts toward this limit, so the "number of letters" is likely to be less than 12000.
While it's technically possible to change this limit I wouldn't recommend doing so. Not only would you have to change the limit in both database and Q2A, but 12000 characters is also quite large to begin with (more than 6 typewriter pages).
The Q&A format is for asking (specific) questions, not for writing essays or having long-winded discussions. If a question (or answer) requires more than 6 typewriter pages of text, I'd argue that it severely lacks focus and should be trimmed down or perhaps split into several more specific questions.