They do vary in diameter quite a bit. I have found six inch at 5.25
to 6.75 inches outer diameter at both Menards and Lowes.
Mathis-Kelly has a little more uniform tube size up to 48" in
diameter. The thickness usually is about the same at about 1/8
to 3/16" thick.
When looking for a tube I will take a coupler to the hardware
store and "size" it to the tubes. This way I can get one that fits.
Always check for roundness. They stack them and they do get
bent.
William