I guess my OW class was one of the larger ones. We had 8 students, 3 of whom were teens, 2 in their 20s, 2 in their 30s, 1 in his 50s. The class was taught by an instructor with a DM assistant who was working on his instructor training. We had 1 evening intro session and 4 weekend days (over 2 weekends) that each included about 2 hours of class time and 2-3 hours of pool time. I didn't feel like the class was too big, because we had plenty of time in the pool for people who had trouble with a skill to work it out with the instructor. Everyone else would just practice things they had already learned, with the DM supervising. Or relax and play around a bit.
For our OW checkout dives, we did a trip through the same shop. There were 3 students doing checkout dives, the instructor, and one certified diver along for fun (trying out new gear and watching his grandson's checkout dives).
There were dozens of people from other dive shops on the last two checkout dives, which were on a Saturday. It was a little too crowded. Trying to do nav underwater while avoiding the fins of divers above you, hmmm...