The same applies to Oracle and MSSQL when using the VARCHAR types like Mendix does.
The size might matter to indices and I believe there is a maximum in VARCHAR of 4000 or 8000 characters. After that you need some unlimited size (Oracle CLOB).
Be careful using unlimited string length in combination with MSSQL server. Check the following link for more information:
https://forum.mendix.com/questions/4238/Mendix%20on%20SQL%20Server%20database%20table%20size%20is%20huge