I am fetching data from MySql Server into R using RODBC.
So in one column of the database is a character vector
SELECT MAX(CHAR_LENGTH(column)) FROM req
You could try to use another db driver such as JDBC. In my experience this has sometimes solved the problem.
Also, try the RMySQL package (current binaries need to be compiled. if you do compile them yourself, request you to please share with the community)
Probably the source of the RODBC package "could" provide insights into the default length limitations if any. (I haven't looked at it yet, but I will soon and post an update here)