I don't know that heighth matters as much as you want to pick them while the head is still tight. Once the head starts to open it's too late. I think about 9" for all is probably good for all sizes.

I agree with Kate, the best quality is while the top of the spear is still tight. I harvest anywhere from 6" up as long as the tip is tight.

I just went out for a walk through on my garden and counted 11 spears up but none were tall enough to pick yet. It's supposed to be in the 70s and 80s for the next few days so they should grow fast. 7 of them were from my Purple Passion and Pacific Purple beds which I like better than the Jersey varieties.

I am watching the spears and agree, that s more important then height. Just seems crazy same height for all (or close) one of the big ones head just started to open. Several smaller varieties are taller and heads are still tight. Seemed odd so I wanted to double check!