Hi,
I do not own the volume knob board but if they use the same knob as for the Up2Stream 2.1 amp board (what I assume from comparing the product pictures) then the diameter should be 6mm. As far as I know, this is the standard for most rotary encoders. The length (axis plus thread) should be about 20mm. Unfortunately, I cannot measure the length exactly since the knobs of my board cannot be reached from outside the speaker box (used my own knob for volume control).
By the way: Nice project. Love the art deco look of the radio 
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Regards, Sven