The 14-16 ft Camper Trailer Cover - Adco is from (4.28 - 4.89m) and the body of a [PENGUIN/SWAN/FLAMINGO] is 4.30m
The size down is the 12-14 ft Camper Trailer Cover - Adco is to suit (3.67 - 4.28m)
If you have a Swan your van is 4.30m it would not fit into the smaller size, which only goes up to 4.28m.
Trying to fit a too-tight cover on a van can cause tearing of the fabric and friction against sharp edges and would void any warranty you had.
The 14-16 is recommended by Jayco for the Penguin so if you have a penguin that would be the best size for you they have straps to tighten them around the body of the van.