Learn How And When To Harvest Broccoli Head Size - The broccoli head typically will get to be 4 to 7 inches (10 to 18 cm.) wide when it is time to harvest broccoli, but don't go on size alone. Size is an indicator, but be sure to look at the other signs as well. Floret Size - The size of the individual florets or flower buds is the most reliable indicator.