This has to do with how mysql will treat your query. See, when you do the LEN() query, it will work the way you want (in that it produces the expected results), but you see, mysql has no way of knowing how many characters are in each testimonial without first checking each row in the database to see how many characters are there. This means for every single row, it will read the entire thing and do a count on the number of characters, so if you had 1000 testimonials, each time you do this query, it will have to scan all 1000 rows.
If instead of doing this, you cache the results of the number of characters and store this in a new column, mysql can tell by querying this number, which rows to return, without having to scan all of them. So say you have 1000 rows and say 50 of them should be turned, mysql will work out instantly which rows they are by querying this number, which has already done the work of the LEN() method in the original query. So in this instance, you would have 1000 rows, but mysql won't have to scan them all and will simply return 50 without going through every row.
In order to do this, you will need to insert the cached number in the new column (call it something like number_of_characters or something like that) each time you insert a testimonial in the database. For the existing records you have, you will have to write a one-off query to go through each row, do a count and insert the value, but this will only need to be done once.
Does that make a bit more sense?