The shortened URL contains 25 characters. Actually it could be smaller but the goal is to generate the smallest QR code. To do so, a restricted set of characters is used. The result is a small QR code version 1.
There is currently no limitation on input URL size. However you might reach some limitation (> 4096bytes) due to the current proxy.
great job! But, you didn't answer nonsapreiche's question about how long the shortened URL will last?
Right! Currently there is no plan to remove shortened URL. Meaning that the service keep shortened URL indefinitely.