It is not possible to set the field to VARCHAR(50000). If you try to do so, the operation will succeed but if you check the field you will see it as a MEDIUMTEXT or another TEXT-family type.
Up to version 1.8.0, the maximum size without the automatic conversion is between 20.000 and 30.000.
What is weird is that you say it doesn't work because the MEDIUMTEXT can store up to 16.777.215 bytes. So if you have updated the QA_DB_MAX_CONTENT_LENGTH constant properly you should be good. You might be doing something else wrong.
More information here: http://www.question2answer.org/qa/39622