Your assets are safe in an NPC station. As for the limit, there is no limit in cubic meters size but the limit is 1000 unique items or stacks of items per hangar or container. If you exceed the limit you can use containers such as station warehouses to have 1000 stacks in each. So you could theoretically have 1000 containers with 1000 items or stacks in each of them for up to a million items/stacks maximum per hangar.
Keep in mind that if your have to repackage or stack items it needs some extra space to carry out the action. If your hangar is too full you may not be able to maneuver things easily.