George
Hi,
I would imagine that with any prostrate bougainvillea, they would have a limited radius of about a 4'spread across the ground or raised garden wall.
If you want to grow them to cover "some long brick tubs that make up a fence", then they might need light pruning to keep their prostrate habit and to encourage them to cascade over the bricks.
I wouldn't attempt to grow an ordinary one for the purpose for what you need, as it will require a lot of regular hard pruning and you will not achieve the desired result.
Ordinary bougainvillea's tend to have strong growth spurts by sending up long thick canes, and they become unruly.
Kind regards
George