Generally speaking, the Open Diameter size, the diameter size increment and the Close Diameters of most worm drive clamps in common use today is dictated by industry standard (SAE J1508, DIN 3017).
When selecting a clamp for use on coolant hose or similar, it is recommended that the clamp be selected based upon the full Open diameter of the clamp. Sizing the Open diameter of the clamp to the hose O.D. will minimize the amount of band tail that will protrude from the clamp after it has been installed.
Resist using a much larger clamp size. The use of a larger clamp size will result in a clamp installation that has a much longer band tail protruding. This could present a snag/cut hazard in some locations. If the tail end is to be trimmed short, then the clamp should be marked, removed, cut short, corners chamfered and edges de- burred before final installation. Do not simply cut off the excess tail as the cut edge will remain sharp and hazardous.
An alternate solution for handling a long tail condition is to simply curl it under with a pair of needle nose pliers. The band material is ductile enough to handle this type of bending. The downside here is, when the clamp needs to be removed, the tail will present resistance unless uncurled.
It is okay to combine multiple small diameter clamps to make a larger one. For example: two(2) 4″ clamps joined together will yield an 8″ diameter clamp assembly. Sometimes there is an advantage having multiple tightening points when working with large diameters not just from the tightening access perspective. Multiple tightening points on large diameters produces more uniform clamping pressure around the connection.