I don't have dimensions, but I can give you an idea of what I used to make mine:
1. Crossman Air pistol from Wal-Mart; removed the barrel housing, and used JBWeld epoxy to secure it into...
2. 2" PVC pipe.
3. Evergreen #189 plastic ribs, found at HobbyTownUSA.
4. I cut the middle section out of a cheap auto funnel for the barrel tip.
5. $8 rifle scope from Wal-Mart, JB Weld-ed onto the top of the barrel.
6. Small piece of Sintra, curved to fit over the cowling, and puttied in place with Bondo, plumber's putty or the like.
7. Custom 'D' connector, made from 2 pieces of Sintra, glued together and sanded down. Someone on this board can make these from aluminum or metal, but not sure.
8. Wood stock, made from small piece of wood, sanded and stained.
9. And finally, a shoulder strap, using shotgun mounts (found in the gun section at Wal-Mart) and an old camera case strap. Not movie accurate, but nice to be able to use your hands once in a while.
I hope this helps you get started!