Choosing the size of ukulele's finger sleeve mainly depends on your finger size and personal preference. Generally speaking, the size of finger covers used by adults is medium or large. In order to determine the right size for you, you can consider the following methods:

1. Finger measurement: Measure the circumference of your index finger, middle finger and ring finger with a ruler or a soft ruler. According to the measurement results, refer to the size guides of different brands of finger covers and choose the size closest to your finger circumference.

2. Try on finger covers: Go to a musical instrument store or buy finger covers of suitable size, try on finger covers of different sizes and choose the most suitable and comfortable size. Make sure that your fingers are tight, but not too tight, so that you can move your fingers freely.