The advantages of small class sizes are so obvious that they don't seem worth repeating.
Like all things you need to look at every aspect of a school but I would take class sizes of 24 as a serious negative.
Sorry but I disagree with all of that. I could cite personal example and say I moved my DC from an indie with small classes to a state primary with 30 - and it was one of the best things I ever did; or I could tell you that the evidence does not suggest any advantage to small classes per se. In fact they are often very effective at masking poor teaching - it is much easier to deal with a class of 14 than one of 30, so you don't need to be such a good teacher. The factors which matter far more are quality of teaching and learning - and it is absolutely not the case that there is a definite link between those and class size. If a class of 24 is a 'serious negative', then I am assuming you would consider a class size of 30 a complete disaster, but my experience, both as a parent of 3 and as a teacher, does not in any way bear this out.
Edited to add: I totally agree with the point made by mad? about the social implications of small classes - my DD was very miserable as there was an 'in crowd' which consisted of every girl except her and one other. Much better when she had more to choose from.