Anyone can take a test, certs on the wall doesn't make one a better tech.
I started with a 40" Craftsman Top-Bottom set when I was in tech school. I still have that box, mostly full and recently added a Snap-On KRSC40 that has all my most used stuff on it. I had a single drawer cart before the KRSC, and it was full before I got everything I wanted to have with me all the time loaded onto it. It was great for $20, even better for the $150 trade in value I got out of it, but it wasn't enough for me.
I'll either buy a bigger roll-cab or an additional box eventually. Thinking about taking a 26" box I have sitting empty at home to set on on my bench for organizing small parts if nothing else. IMO, storage is like horsepower, no such thing as too much.
I'm lucky, where I work doesn't put limits on box size. Most of us have a 40" or bigger cab + a cart, or multiple boxes. One guy has a 26" set, a cart, and actually brought in a shelving unit on wheels that's equal easily to a 56" cab, but way deeper than any on the market. I think a 40" is the bare minimum for someone working full time and should keep you within anyone's limits on box size.
As far as working in the industry, especially at flat rate goes, if you can't make flat rate on a job you've done more than once or twice, you need to work on your methods and put down your hand tools. I'm not that fast and I can pull flat rate on most jobs or be close. I'm fortunate enough to be on a split-pay system (one hourly rate for the flat rate hours billed, less than that for any additional time) in an independent shop, I work on EVERYTHING from the early 50's on, from Honda to HD truck. I don't know a single one make, dealer tech that isn't 150-200% efficient.