my oldest 2 went ot a small school There was one infant class and one junior class. Total in the school = 40
there were pros and cons
good points:
family school, everyone knew everyone, close relationships between teachers and children, the vertical grouping gave flexibility. teacher pupil ratio was great and kids got lots of personal 1:1 attention and the teachers all really knew the children well
bad points
everyone knew everyone! Could be claustrophobic.
you had the same teacher for 3 years in infants and 4 years in juniors, so any less than great teaching gets intensified, and if you don't get on with your teacher you are stuck with them
one teacher in our school was very average and I would not have been happy for him to have been the only teacher mine had for 3 or 4 years (he actually job shared so it wasn't so bad)
only 2 or 3 teachers puts a lot of pressure on the teachers to do everything, and if you (eg) don't have anyone who has an interest in music or pe, then you don't get any of those extras (eg a choir/football club) that you get in a school with more teachers.
There is no football team in a small school, - just not enough kids, and if you are a 7/8/9 year old boy who wants to play football, that is an issue
Also my ds is above average and by chance in his first year all the other children in his year group (4 others) were having some sort of support or help. Children work well with others who are at the same level, and someone a bit ahead can spur them on. But he didn't have anyone, until a new boy arrived - see below!
but the biggest issue was friends. My ds had one other boy in his year group, he was ok, but then he left and a new boy arrived. They didn't get on. it was very intense unhappy relationship. They had to work together and play together. There were older boys around, but that also had problems, at Y3, my ds played a lot with a Y6 boy, which was ok, but didn't always work. He really struggled with this and he wasn't the only one. One mum I know finally moved her daughter in Y5 to a bigger school, because there were only 3 girls her age and she didn't really have a friend. Within days at the new school she had made friends and was much, much, happier.
On the other hand, my quiet dd went into reception in a year group with 8 children and there were 3 girls in the year above too. They made a group of 4 friends who were very close and had no competition so it was a very happy group. She also had a close relationship with her teacher and TA, who knew here well and looked after her when sometimes she was a bit little for school (she always needed reminding to go to the loo) It was a lovely start for her.
We moved house and found a one form entry school as we didn't want a big school and I am glad we made the change, my kids are doing much better in lots of ways but we also at times miss the small closeness of the village school.