Can't comment on the length of the studs. You won't want studs any longer than you need for OEM fitment.
Only reason to use longer studs is if you are going to be using spacers.
I upgraded my lug bolts and they were 27mm under the head of the bolt to the end of the threads.
Do a search on here for wheel studs, the answer to the required length will likely be there.
Heres a good one
http://www.zpost.com/forums/showthre...ht=Wheel+studs