Zodlebud, you have a correct observation that class sizes at the King's School are small. That is one of the reasons that allows for good pastoral care.
I think if some one is seriously contemplating joining the school then rather than listen to someone speculating on it I would go and visit it. If you phone the school they will be more than happy to arrange a tour.
Yes with a small school there are not the facilities that some can offer and GCSE choice is more limited to what some bigger schools can provide.
However, it really comes down to what each individual family is looking for. Not everyone wants or needs every club under the sun and the choice of twenty plus GCSEs.
For such a small school those that are sporty (of which all mine are) were given lots of opportunity to participate in football, athletics, table tennis, netball etc.
As already said I would draw my own conclusions on a school by visiting it. There are lots of different schools out there, each offering their own unique setting. However if someone is moving to Harpenden then you may not have the choice of schools as they are very difficult to get into, limited to very small catchment areas.
The King's school may be small, but it offers a rich experience to all those families who go and have been there.