My custom made trailer frame is 61" wide by 117" long with a 48" long A-frame tongue.
A little oversimplified:
The front and sides were made with 2"x2"x1/8" square tube.
The cross supports on nominal 24" centers are 2"x2"x1/8" L shapes.
The A-frame tongue is made of two 2"x3"x1/8" rectangular tubes.
In the four corners we used 1/8" flat plate triangles to help bolt the floor to the frame.
I had a local welding shop custom make the trailer. To my pleasant surprise it was completed two days after I gave them my plans.They buy steel for less than I could have. I supplied a Dexter #9 torsion axle that was built to my specifications. If I were to build another trailer I might use a Flexiride torsion axle, instead, which has adjustable start angles.
If I ordered steel from MetalsDepot.com:
T12211 2 X 2 X 11 GA (.120 wall) A500 Steel Structural Square Tube $68.88 each $137.76 for two 24'
A12218 2 X 2 X 1/8 Steel Angle A-36 Steel Angle $29.60 for 20'
S111 11 GA. (.120 thick) Steel Sheet Hot Rolled Steel Sheet $20.60 for 1'x2'
T13218 3 X 2 X 11GA (.120 wall) A500 Steel Structural Rectangle Tube $70.20 for 12'
Sub-Total: $258.16 not including shipping