The plastic knobs fit 6.35mm (1/4 inch) shafts. The knob is 20.3mm (approximately 13/16 inch) diameter and 15.6mm (approximately 5/8 inch) high. About the same size as a Nickel.

Shafts that fits all my poly variable capacitors are available in 12mm or 14mm length and fit these knobs.

12mm x 6.35mm Shaft

14mm x 6.35mm Shaft