The knob is 34.8mm (approximately 1 3/8 inch) diameter and 18mm (approximately 11/16 inch) high. These fit 6.35mm (1/4 inch) shafts.

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