Pick the option closest to the size you use.
How do you define "primary shelf"?
I have four shelves. Left-middle (40px), bottom-left (152px), bottom-right (152px), and bottom-middle (40px). They each have their specific use. None are what I'd consider "primary".
My non-autohide shelves are 28px. I have found that on 15,6" screens larger is OK, but smaller laptop screens don't just give more screen area, but look better as well. My autohide, left-side shelf is 40px as I recall.
If you were to ask me for a default, I would clearly say 40px or less.
I agree with davek, define "primary shelf", as I don't consider either one to have any significance over the other one. I only have two coz I needed some things to be bigger than others. So one is 32 pixels, the other is either 100 or 104 (due to the rounding bug in the pager). Which puts mine outside of the range at both ends of your scale. lol