Leashed dogs are usually limited to campgrounds, paved roads, picnic areas, and some beaches. Most dirt and backcountry trails are off-limits, though it varies by park.
Most unpaved trails and the backcountry. A common rule of thumb in California is that dogs can go where cars can go.
A handful of California state parks are more permissive, so check the specific park before you count on trail access.
6 feet maximum. Retractable leashes are often not allowed.