1/8" ID, somewhere either earlier in this thread or somewhere else somebody suggested putting 1/8" ID plastic hose inside a dip tube, which seemed to work IIRC. Just putting 1/8 stainless, which would have thinner walls would not cause much restriction because beer would flow around the outside as well as the inside. If I did the math right a solid stainless rod about .1" or .11" insert in the tube would be about the same restriction....
((((.125 ** 2) * pi) - (((.125 / 2) ** 2) * pi)) / pi) ** .5 = 0.108253175
a 1/8" four foot long stainless rod is $3.36 at the first place I checked.
At any rate, my system works with 8' of beer line so I have no need for any of these solutions, I'll leave it for others to test

If I ever set up a portable system I may try this to see what happens.