In this official Apple support document, Apple lists the limits on calendars and reminders, contacts, and bookmarks.
For example, here are just a few of the limits on contacts:
- Total number of contact cards: 50,000
- Maximum size of a contact card: 256 KB
- Maximum size of a contact photo: 224 KB
- Maximum size of a contact group: 256 KB
[Via 512 Pixels]