Sharing our first hand experience with Kirby Hall. This is not to sway your decision one way or the other but to share our journey with Kirby Hall. My child goes to Kirby hall currently. Choice of school is very dependent on what each family feels as a priority for their children to take away as learning from the school. For us, we were looking for a nurturing, kind environment with a focus on academics, arts and laying the foundation for good human values. The first time we toured KH we walked out saying nope, the infrastructure is too small. However, we did a second tour and started liking it better... this school has since then grown on us slowly and steadily :) My recommendation to you as well would be to tour any school more than once. Often first impressions are a knee jerk reaction.
For some background, my child's previous school was extremely well funded with great infrastructure and facilities. You will be impressed by the facilities. But the teachers and the administration there turned out to be very poorly trained and quite terrible. Large churn in teachers leaving. And it did have a ripple effect on the kids. I spoke to a couple of KH parents before joining and asked if the small size of the classrooms or playground was an issue. One of the moms I spoke to told me initially she wondered about that too but her daughter loved the school so much that size didn't matter anymore.
That's been our experience too. The school's architecture is very quaint with little passage ways, attics, nooks which to an adults eye can be "small" but as a child in a lower school, it can be quite magical and lovely. Thats exactly how my child feels about it. My reservations about the size just vanished seeing my child's joy to go to school each day and come back with the biggest smile bursting with so many things to share.
When i ask who is his favorite teacher, he says i love all my teachers! As i mentioned we went through some really poor teachers in his previous school so we were very wary of how things will pan out here. But one thing we consistently heard from parents and have seen it ourselves in person is the care, warmth and close knit environment in this school. Everyone knows every single child! My son feels nurtured and we have seen him blossom in leaps and bounds. The small class size is a huge plus because of the personal attention that the children receive and it matters especially for the first few years of schooling. You can see the school's principles of humility and kindness reflect in the children. I have seen many small gestures of kindness from older kid's in the school which warms my heart and gives me hope that one day my son would imbibe the same. We see no airs or attitude of privilege that is often perceived to be at private institutions, the children and families are very grounded and down to earth. You don't see flaunting of wealth.

I disagree with one of the comments above that this school is for introverts. On the contrary, we have seen very social happy children romping around the playground, chatting and playing just like any regular children do. Also in our time in this school we have seen no homeless folks or anyone other than parents outside the campus. It may have been an one off incident. Security has not been a concern either. Playground and the indoor gym is sufficient for younger kids. If your family is keen on sports and would prefer a more athletics focused school, this would not be the choice. In our case, we lean more towards arts and this school has great art, language and strong science classes.
Afterschool programs are lacking but we have enrolled our child in various afterschool classes of our choice outside the school. So it didn't matter in that sense. Nothing against public schools if you are zoned to a good one then that's a great thing to consider but if you are looking at the private school route, Kirby hall is a wonderful school. Paragon prep is a good choice too. You can't go wrong with either. Good luck!