Having a nice set of graduated test cylinders can come in handy when fermenting, distilling, blending, and diluting spirit. If you're looking for just one a 250ml size can be a nice size to float a hydrometer.
I like to use a 250ml to measure specific gravity during fermentation and measuring proof during distilling with a hydrometer. A 500ml is a great size for making cuts. 100ml works well to determine dilution rates or blending spirits.
Capacities available: 5mL, 10mL, 25mL, 50mL, 100mL, 250mL, 500mL, 1000mL, 2000mL. Most sizes are accurate +/- 2ml. Larger sizes are +/- 5ml.