In response to the OP, if you fear a large school may not suit your DD, then probably SHS is not for you.
That said, the class size is still smaller than state schools, and if they take more children they will also take on more staff, so the ratios should be no different for individual forms.
Having a large cohort is another matter entirely, and creates more competition for being in top sports teams (especially with the numerous sports scholars!) and music ensembles etc.... Unless you had a very confident and accomplished child, they may not get their chance to 'shine' in the same way as in a smaller school.
But then again, that may not be important when considering the factors which you value in a school. Once you know what is most important to you and your DD, then you find a school that fits best (sadly, there is unlikely to be one that ticks all the boxes...no school is perfect!)
I know people who have been happy with the school, others who have chosen it because they feel they really didn't have another choice (better than their local state school), others who were tempted by their generous scholarship offer (or staff discount offer) and others who removed their children because it wasn't what they thought it would be. There are no guarantees - the type of children in a particular class/year group is completely unpredictable and some personalities create a delightful, harmonious year group while others do not.
Good luck if you are making a choice on what to do for your DD ...and then good luck to her in passing the exams to get in where she wants to go!