3-form is quite large for london, maenaed. Islington (our borough) has no schools larger than 2-form intake, and I suspect others are similar.
But I don't think the absolute school size matters as much as the way it's all handled by the school. I was expecting a 2-form entry school to feel v. different to a 1-form one, but I can report that it doesn't really.
I would ask:
Do they mix the classes up as they go up the school, and if so on what basis?
Are the classes done on ability or age-streaming?
What arrangements do they have for children to mix within the year group? Four form is too big for them to do eg. joint PE lessons, so would be interested to know how it is handled.
What arrangements do they have for children to mix across year groups? it strikes me that in a school with year groups of 120 the opportunity to mix with kids from different year groups (valuable imo) might be limited, so would want to see what the school's response is to this.
How do they deal with year-based/curriculum based activities, eg school trips? If all Y4 classes are doing the same topic, do they take all of them to eg. the British Museum at the same time? Whatever the answer, the question is how do they get the balance between sharing activities across the year group and preventing the groups getting too big?
Also think about what kind of parent you are. Are you academically pushy, or do middle-class boden-wearing pushy parents get right up your nose? What are the main secondary school destinations for the Y6 kids, and how does that fit with what you envisage for your children?