SQL injection boolean-based blind test - How to get the length of first table name?

I'm pretty new to SQL & most of my exposure is through Python e.g Django ORM or sqlite3 i.e. not proper SQL!

I'm trying to solve a problem in order to learn & I've got to a point where I've tried every permutation of every possible query I could think of / find on the internet (within reason) related to what I'm trying to do, and am obviously missing something fundamental.

The task I'm struggling with is to form a query that can dynamically check the length of the name of the first user TABLE in the database (which I know has a parameter 'secret').

I can do this for the actual database name no problems with

    ...' OR LENGTH(DATABASE())=%variable%;

But when it comes to the TABLE_NAME I can't seem to form the right query as I said. This is what I've tried:

    SELECT LENGTH(TABLE_NAME) FROM information_schema.TABLES WHERE TABLE_SCHEMA = 'databaseName' AND TABLE_TYPE='BASE TABLE' LIMIT 1;

This works and returns the correct length of the correct table name. However, doing something like:

    SELECT * FROM 'test_name' WHERE secret='' OR (SELECT LENGTH(TABLE_NAME) etc etc LIMIT 1)=%variable

/

    ' OR LENGTH(SELECT table_name FROM information_schema.TABLES WHERE etc etc LIMIT 1)=%variable

both fail miserably!

I made an example sql server to test:

I didn't get any further than I've described above without error messages :(

Where am I going wrong??

Please help!!! My brain's about to explode!