What's the difference between petit and grand taxis?

The short answer

Petit taxis are city-only and metered. Grand taxis are intercity and shared.

Petit taxis operate within one city — metered, max 3 passengers, colour-coded. Grand taxis are intercity — 6 passengers, fixed fare per seat, they leave when full. Grand taxi stands cluster near the bus station (*gare routière*). If you don't want to wait, you can buy the empty seats.