It all depends on what space there is to build a garage. I have four garages at home and there is still not enough space!
The minimum I would go for is a metre bigger than the caravan on all four sides; 2 m is even better, especially if one is not good at reversing!
Not sure what type of garage door you will use; some require a front wall with a narrower section for the garage door. In this case, go for the 2m either side of the caravan. For the roof… again you might have to allow for the type of garage door, and whether it has a door jamb or not. A roll-up door has no door jamb, so add between 20 and 30 cm to your caravan height to clear the pop-top roof.
Once you have the caravan nicely positioned inside the garage for the first time, put bricks behind the wheels, so that the next time you park the caravan you will encounter this resistance rather than the back of the caravan pressing against the wall!