i think i used 1 1/4" electrical conduit (keep in mind that in the conduit and pipe world what it is called isn't exactly what it measures).  i recall the tubing to be close to 1.5" outer diameter.  i've seen smaller diameter tubing for frames but the large diameter is stiffer and is easier on the hands when carrying.

my design included having the frame bend up and then back down to create the oar towers - just drilled and plug welded in the proper size of pipe to hold an oarlock and bob's your uncle.

- sheik