Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the average fleet size for the Shunter type introduced in 1959?

### Inputs:
table_name: table, columns: ['Class', 'Type', 'Introduced', 'Fleet Size', 'Numbers']

### Response:
SELECT AVG Fleet Size FROM table WHERE Type = shunter AND Introduced = 1959