They would do that in my town, where I grew up, because we had a (big flat) area (that was flat), so (1)there was a lot of room for the balloons to be(2)blown up/inflated and to launch/start.
Use either of 'big flat area' or 'big area that was flat'
Lots of sports have special words to refer to things in that particular sport. It may be that hot-air ballooning has special terms to describe the actions to which you are referring. But I don't know that, you would need to ask someone who participates regularly.
Student or Learner