In Dart, you can get the size of a hash (or map) using the .length property. Here is an example code snippet:
Map ages = {'John': 30, 'Mary': 25, 'Bob': 40};
print(ages.length); // Output: 3
In the above code, ages is a map with three key-value pairs. The .length property returns the number of key-value pairs in the map, which is 3 in this example. You can replace the key-value pairs and variable name to your desired hash to get the length in Dart.