Sandy, there are lots of reasons why people can't have animals inside. Being a wanted outside kitty is better than being an unwanted feral. Yes, indoor kitties generally have longer lives, but that doesn't mean all cats must be indoor pets, and it sounds like these two will be well taken care of. We keep our cats inside, but there's too much traffic in suburbia to do otherwise. I know this is a "hot button" topic.
I checked on Amazon to see if I could find you an example of the type of door I was talking about and found one that I thought was an even better use of technology... the door is keyed to the cat's microchip! Having the vet 'chip your kitties when they are spayed is easy, and a microchip means they can be ID'd and returned to you if they wander off.
https://www.amazon.com/dp/B07DWTLGPG/
As for door size, the size needed is going to vary depending on the size of the cat, and you don't really have a way to know that yet, so I'd go with a larger one.
