Exactly where can a stowaway hide in a wheel well?
For the 737, I think this rather sad photo provides one answer:  



(Source)


  The body of a stowaway was found in a Kenya Airways flight from Kinshasa in the DRC on Sunday.
  The body was found after the plane landed at the JKIA, preliminary reports indicating the person froze to death.


And, as you can see in this photo, the ledge there (on the left forward side of the main wheel well) is indeed just wide enough for a slim person to contort themselves into:  



(Source)

But, as you can see at the end of this video, there certainly isn't a lot of room for the stowaway's legs.