There is not a fixed size of any kind of shelters for keeping crayfish. However, you can use some flower pot in your tanks. The size of pot that you choose depends on the size of your tanks and shrimp. I previously used small flower pots (height: 10 cm, diameter: 7 cm) for Swamp shrimp (Macrobrachium nipponense).

you should use PVC tubes of various diameter and some bamboo pools