To know when broccoli is ready to pick, run your finger over the green central flower head; if it feels tight, you can delay picking, but once the buds start to loosen, start the harvest. Broccoli florets can be cut 1 to 2 weeks before they mature. Broccoli heads can be as large as 4 or more inches across, but smaller heads are very flavorful. Cut heads with an inch or more of stem attached; both the head and stem are edible. Whenever you harvest, it is important to leave 2 or 3 inches of the stem behind. New buds will form on these stems.
For more information on growing Broccoli, check out this great guide from Harvest To Table: