I chose a 2-form-entry primary for my DCs, in preference to a 3FE school a similar distance away. Both were rated good, so the school size was one of the main reasons for my choice. I thought my sensitive DC would feel less overwhelmed in a smaller school, and it would help to build his confidence. The smaller school also felt less crowded at break-times, because it had more playground space per-child.
I haven't regretted my choice. My DCs know the names of nearly everyone in their school, as do the teachers. They're able to have whole-school assemblies, and Christmas Plays that give everyone a small part to play. My DCs are developing into confident, outgoing young people, who are very comfortable at school.
However, as they grow older, I'm also looking over the fence and seeing the relative advantages of bigger schools; not just the 3FE school I originally looked at, but also the 4FE school slightly further away that I never even considered.
Larger pupil numbers bring more funding, so they have additional facilities, such as astro-turf pitches. Their bigger pool of staff and keen-to-volunteer parents mean that they can offer much more by way of extra-curricular activities.
I'm noticing a direct connection between that and their performance in borough sports competitions. The parents at my DCs school routinely complain about lame PE, minimal after-school activities, and a defeatist attitude to inter-school competitions (not just in sport; we tend not to compete well in other activities too).
As my DC1 is pretty sporty, and competitive, I can see that he would have got a lot out of a bigger school, that could perhaps afford a specialist PE teacher, rather than just a couple of members of staff who are keen to coordinate those sorts of activities, but who are not particularly good at it.
Anyway, I've waffled on a bit, but I'd be interested to hear other people's perspectives on their choice of a small school over a big one, or vice versa.