Handling large input sizes is crucial in competitive programming, as even efficient algorithms can fail if the input/output operations aren't optimized. First, use fast input/output methods like `scanf/printf` in C++ or `sys.stdin.read` in Python to speed up reading and writing data. Efficient data structures like segment trees, binary indexed trees, or hash maps are essential for managing large amounts of data. Also, avoid unnecessary computations by using memoization, caching, or preprocessing. Precomputing results that can be reused or skipping unnecessary loops can reduce the overall time complexity and prevent timeouts on large inputs.