A float does two jobs: it suspends the bait at a depth you choose, and it tells you when something has taken it. For species that feed in midwater it is the difference between fishing and waiting.
Shape follows the water. A slim waggler sits low, shows the faintest bite and suits still water; a fatter bobber rides a chop and holds its position in a flow, at the cost of some sensitivity. If a fish is taking and letting go, the float is usually too buoyant.
Set the depth to just above where the fish are holding rather than level with them. A bait falling gently into view draws far more takes than one hanging in front of them.