This came up during the rebuild of a very old legacy application. The previous app used varchar(255) for most columns, but this seemed a bit wasteful.

To get a better idea of how large things needed to be in the new design, I wanted to know the the longest value stored for each column.

Thankfully MySQL has the length function that can be used for this. The following query qill return a single result containing the longest value, as well as its length:

SELECT <column>, LENGTH(<column>) as col_length 
FROM <table> 
ORDER BY col_length DESC

Just replace <column> and <table> with the column and table name to query.