MySQL: check max chracter length (ERROR 1118 (42000): Row size too large)
According to this, your problem is not with VARCHAR fields. However:


SELECT id, LENGTH(field1)+LENGTH(field2)...+LENGTH(field_n) AS total_length WHERE total_length &gt; 8126
Can be done using 2 queries for each field. (This can be done in a complex query if you want to use INFORMATION_SCHEMA):

SELECT MAX(LENGTH(field1)) INTO @max_length1 FROM table_name;
SELECT * FROM table_name WHERE LENGTH(field1)=@max_length1; 



Note: Increasing innodb_page_size's value may be helpful in your case. 

Edit

To generate all the queries with the help of information_schema, run the following queries:

CREATE TABLE queries(field_name varchar(255), query1 varchar(4000), query2 varchar(4000));

INSERT INTO queries SELECT column_name, 
CONCAT('SELECT MAX(LENGTH(',column_name,')) INTO @max_length FROM table_name;'), 
CONCAT('SELECT * FROM table_name WHERE LENGTH(',column_name,')=@max_length;')
FROM information_schema.columns WHERE table_schema='test' and table_name='myisam_table';

SELECT * FROM queries;


Note that you have to run query1 then query2 from the result. DO NOT run all query1 then all query2!