I flew Trenton to Raleigh-Durham on Frontier in December of 2019. The airport is very small. There are two "gates" which share an exit door to the outside where you board the aircraft via stairs or ramp. The gates share a waiting area that takes up about 80% of the lower level. Security and an airside bar take up the other space. Baggage claim is in what was once a mobile-trailer type building. Similar to offices at a construction site. There are parking lots right outside the terminal, but depending on how busy they are they might be filled. In that case they have overflow parking further down the road in a big grassy (muddy) field. A shuttle runs back between.
When we flew out the close parking lots were full, and the field was 75%+ full. There were two planes worth of people in the waiting area, so it too was crowded. We were a party of 4 (2 adults, 2 children 8 & 11), and did not purchase pre-assigned seats. I just checked in online as soon as check-in was open. We were auto assigned seats near the back of the plane. 3 seats together, and 1 across the aisle. ie. Row ## A, B, C and D. On the way back to Trenton, we were assigned same row again, but C, D, E and F. I've read if you check in as soon as possible, you have a better chance of having seats assigned together. It worked out for us, but we were prepared to be somewhat separated.
The bigger concern is what happens if your scheduled flight gets cancelled. Depending on how many flights out of Trenton there are per day and how full your flights was, getting booked onto another flight in a reasonable amount of time could be challenging.