In addition to the opinions given by the doctors, I would suggest you may share some of your dad’s past photograph, when he had teeth, with the doctor. This may help in a way, if that’s the kind of the teeth set you wanted to have. I’m sure the doctor would help you give an aged looking teeth according to your Dad’s age. This can improve the denture.

I totally understand your concern. Your father was teeth less for quite sometime. Sudden placement of dentures will definitely give a fullness to the cheek and change the appearance. That is because you are not used to see him like that. You are habitual of seeing him with the cheeks relaxed due to missing teeths. It is normal. You will get adjusted to this in a month or so. Uneasy discomfort, difficulty to eat, swallow, speak, more salivation are all normal and it shall all fall in place with time. Your concern about the length- the front two teeth are usually 2 mm bigger than the adjacent tooth. If it is still disturbing kindly visit a dentist to get it sorted.

Generally we do trail of denture before final. In trail we set teeth on wax , and it can be moved with in limit to get desired position accepted by patient.  The problem your father is facing is most common thing & most of the patients facing same.  After bearing denture there is change in facial profile,  it is due to denture, & it will be fine only if your father continue bear it for at least one month.