How big should I let my legumes get to get the most nitrogen?

If properly inoculated legumes begin fixing nitrogen early on in their life cycle. That being said, the amount of nitrogen created is related to the dry matter production of the plant (ie, how big it gets). When a legume begins to flower, it's vegetative growth has halted and dry matter production will not increase much more. To get the absolute most nitrogen out of legumes, allow them to reach early flowering. Do consider how this will impact your overall system: is maximum nitrogen production your goal (cover crop) OR is your goal quality forage production (grazing)?