There are some restrictions (caused by lengths of some columns in database).
Taxonomies
- Taxonomy name – 32 characters
- Term name (and term slug) – 200 characters
Posts
- Post type name – 20 characters
- Post name (slug) – 200 characters
- Meta key – 255 characters
Options
- Option name – 64 characters
Comments
- Commenter email – 100 characters
- Commenter url – 200 characters
- Comment meta key – 255 characters
Users
- User login – 60 characters
- User nicename – 50 characters
- User meta key – 255 characters
- User email – 100 characters
- User url – 100 characters
- User display name – 250 characters