The slope (how steep an area is) should be no greater than 1:50 (1.15º) in any direction, which is nearly flat. Grading an area as close to level as possible will reduce the likelihood of something rolling away. If a ball can roll in any direction, so can anything with wheels (chair, stroller, cart, luggage, etc).
Exception:
If a nearly flat space isn’t feasible, slopes no more than 1:20 (2.86º) in any direction are adequate for most people, though expect things on wheels to roll away.