what size tubing for frame

Heres a truck from about 5 years ago not sure its where abouts now but it had a tublar rear frame ot sure if it was 2.5" but definately something around there and it was a single rail rear frame,,pretty trick truck also had the torsion bar hydro setup for the front..but it can be done with a single rail,just look into the material you are using i bet it will be costly

http://www.socalcustoms.net/Shows/ForbiddenFantasy/31.jpg

and you can look in my profile for a little rear shot of my truck and u can see part of my 1.75" frame

My rear frame section is made out of 2 1/2" x1/4" square, it's been like that for 5 years and I've never had a problem with it. I'd have no problem using it under the cab of a mini. I'd be concerned about welding ability more than anything.

at bit off topic but knowing that the DOT is kind of a pain in the ass, how does one get a proper inspection in order to drive it on the road if one does a full custom frame? I know from an engineering stand point that if welded properly, a new tube frame is 100x stronger than a regular stock frame, but tell the DOT that. Just was curious.

As far as the DOT goes, I'd say that what they don't know won't hurt you. Otherwise you might find yourself having to register it as "Special Construction" or for show only. I'm not familiar with your laws though, so I don't know if you'd have to go through some sort of mandatory inspection involving the construction of the vehicle.