I tried to grow rhubarb in a 50 gallon container. It worked for a couple of years then it didn’t. I finally gave up, divided and transplanted and its growing great! The only thing is, I’ve forgotten how big they are supposed to be ( how many stalks) before you harvest.
I don’t think size makes a big difference. I’ve used them at about 1 inch diameter, and at more like 3 inches diameter. They should be juicy and not so mature they are stringy. I usually cut them when the leaves are fully unfolded, but I don’t always wait for the leaves to grow to full size. On my Victoria rhubarb, the younger shoots are redder and have more sweetness, and the older stalks are more green, more stringy and seem to have less flavor. My unknown variety has much redder juicier stems even when the leaves are full size…
You may also want to hold off on harvesting any this year if you just divided and transplanted in the Spring, unless it is already putting out nice fat stalks. If you harvest when it is still smaller you will delay the maturing of the crown and reduce future production.