The size of the pipe depends on your watershed and the vegetation on the watershed. Your local county NRCS should be able to calculate the pipe size. You need a primary overflow and secondary overflow. NRCS has a 70 to 90ish page manual on how to properly build a pond. You can download it.

If you used a vibratory smooth roller, you possibly could have leaks between the layers. How thick were the layers that you compacted? You have to watch that you don't make those layers too thick or they won't compact properly. Ideally a sheepsfoot roller (vibrating or not) is the preferred compaction device.

Just remember that if the spring that feeds your pond is below the full pool mark, then the spring can also take water out of your pond if the water pressure in the pond is greater than the water pressure in the spring. Springs don't have one way valves on them.