While I think this question should have been answered by a simple Google search, I can’t, realistically, vote to close under any of the existing options. As such I choose to offer an answer instead, in order that, hopefully, it won’t be asked again and, if it is, subsequent questions may be closed as duplicates.
The maximum length of a ‘mediumtext’ field, in MySQL, is:
A string with a maximum length of 16,777,215 characters.
Or, according to the docs:
L + 3 bytes, where L < 224