I believe the expression you're looking for is "cockstunned" Thingwithfeathers.... I'm told I have a good cockstunned expression, even though I'm a pretty experienced gal when it comes to size.
I decided to start at the last page in "Sex With a Large Penis" (651 pages as of today), and work my way forward. So, I'm seven years too late on this thread, but find it worthy of comment that no one parried your "cock stunned" expression with the alliterative "cock shock."
p.s. A google search, and the Urban Dictionary, reveal, sadly, that I have not coined an original phrase. However, I did find
Shocked by the Cock in the search results.