Both happen. Although bench cushions are a popular trend right now, there are thing to consider,
- Fabric or leather can stretch.
- There is every likelihood they will wrinkle.
Max Length: 86″
See here the progressive nature of bench seats.
Not all fabrics or leathers will stretch to this degree, but your customer needs to understand that wrinkles on a long bench cushion are a strong possibility depending upon the cushion fill and material. Maintenance is required.
Now you know.