Sill oversize is a cumbersome and broken feature within the door and window dialogue box.
Without sill oversize selected, the sill sits in front of the door so it cannot be opened. The door always sits on top of a sill which is there to provide a shelf for water to flow away from the building. Therefore, sill oversize shouldn't be a feature, rather a necessity.
The main issue however, is when sill oversize is toggled. This causes any casings on the door to be shifted down as it is linked. This renders the door unopenable, unless your door opened inwards (which for external doors with casings, never do).
There is no reason why the two should be linked as they are listed as different features. This causes confusion and took a while to tease out. Could one of the developers rectify this?