While you won't be completely free to explore them, we're being promised huge areas that are handmade.
It seems like exploring everything there is to see in Star Wars Outlaws will be a massive undertaking. In the latest issue of Edge Magazine, there's an interview with creative director Julian Gerighty, who had some fascinating details to offer.
It turns out the planets in Star Wars Outlaws (four of them have been revealed so far, and a fifth heavily implied) will be quite big, and better yet - they will be handcrafted instead of working with procedurally generated environments. As far as size goes, the planets can be compared to "two or three zones in Assassin's Creed: Odyssey" and travelling across them will be a "journey even on a vehicle that's fast moving".
You won't be entirely free to travel around though, as there are pre-determined areas for take-off and landing.