What is the ideal volume to prevent activity loss from freeze-thaw cycles when preparation of Z-VAD-FMK solution?

The best practice is to aliquot in 10μL to 20μL volumes.

Stability: Z-VAD-FMK is sensitive to repeated temperature fluctuations. Smaller aliquots ensure that you only thaw exactly what you need for a single set of plates.
Concentration Tip: Most researchers prepare a 10μL or 20μL stock. A 10μL aliquot of a 20μL stock can treat 20 mL of medium at a final concentration of 10μM, which is usually sufficient for a few 6-well plates or a 96-well plate.