We booked the group tour and were in total 13 or 14 people plus our guide. During our tour we saw big busses with 30+ people so I am pretty happy that we booked that tour. I think this group size is totally ok.
The tour includes a lot of driving (from kiev to the exclusion zone and of course in the exclusion zone). Our mini-bus was very comfortable so that was no problem.
We also booked the lunch which was a simple three course ukrainian meal (salad, soup, main, dessert). Totally adequate for the price and the circumstances (you are in an exclusion zone!!!).
Our tour guide was very nice, laughed a lot, showed us cool stuff, made sure we are safe and told us everything we wanted to know. Thank you Oxana!
All in all it was a crazy unique experience.