Large secondary schools

it depends on the child.
DD started off in a 12 form secondary, but didn't cope. She then went to a tiny private school with 18 per year group and she thrived there.

DD didn't like the hustle and bustle of the larger school, and did feel lost and 'unknown'. There was a constant stream of cover teachers and rubbish, generic reports. Maybe it was just a crap school though.

DC's school is 16 form entry.
The reality is that your child doesn't necessarily notice the size of the school as they will only concern themselves with their immediate groups. so, for example, in KS3, everything bar maths and PE is taught in the same class (which worked out for one child who made a friendship group that she stuck with up school, and didn't for the other who hated everyone in his class and sought out friends elsewhere). So I guess big=more chance for friends!

there is definitely more money and resources available and a larger pool of teachers to share knowledge and ideas and cover each other where necessary. I also genuinely feel that each child is known individually. DD developed a disability last year, and I've been so impressed by the infrastructure that swept in to make sure she was supported.

I'd pick a big secondary school over a small secondary school every time.

DD's school was a 9 form entry.
In the first year they split the year into 3 equal ability thirds, and they only had lessons within that third (so two other classes).
Then in the second year they were split in half, but differently.
Third year was split into bands depending on mini options.
GCSE year was split into bands based on triple science and then ability.

The splitting meant they could get to know other pupils without getting 'lost'.
Later the banding meant they could set appropriately for core subjects.
The larger side meant a bigger range of options could be provided, plus more extra curricular.
I never felt my DD was lost from an academic point of view - the school looked out for everyone whatever ability.

However DD who is a bit sensitive to noise, and is petite with poor motor skills so wobbly on stairs, really didn't like lesson change over at all. She found it very stressful.

My DDs go to a large secondary school. Very happy there and settled into their niche within the school well. We looked at a small school as our other option but it just didn't offer the breadth of subjects and specialist teaching that their school does. School they attend is a comprehensive but groups maths+science and then English+mfl+humanities by ability, six set levels in two language streams (so 12 class intake) so children are taught with others at a similar attainment level. Other subjects are mixed ability. DD1 is doing a very specialised art related subject A level that she wouldn't have had the opportunity for at GCSE at a smaller school. Pastoral care is good, no worries about that. They initially found the size of the school buildings etc daunting but once they found their way around were fine. They had support from older pupils they knew from our church (faith school) which helped. And though it is a faith school it is also a very mixed school (other faiths, socioeconomic backgrounds, ability etc) which I think is great.
For context I went to a two form entry grammar school. We could have moved back to my hometown, and I think they'd have got a place at the grammar school there, but I honestly feel that they have a better / broader education than I did. No regrets.

Lots of pros - good structures in place, and often better choice of subjects for gcse etc, more choice of friends.
Can be a bit daunting, but I sort of feel it will hopefully stand my kids in good stead for the big wide world.
Well funded, our state 12 form entry academy has great facilities and technology

Down side - toilets are gross ☹️ But that may or may not be to do with the size!

My eldest's school had an 8 form intake - for the first couple of years they were treated as 2 lots of 4 forms for setting etc. It wasn't until GCSE options they were mixed with pupils from the other 'half' and that was for subjects with smaller take up. I know 12 form entry is larger but they may structure the first few years in a similar way.

I think people who went to small schools are brought up hearing a lot of ‘aren’t you lucky’. I certainly was certainly told that a lot.
Personally, I think it’s a bit of a lie to cover up the fact that bigger schools can offer so much better choices and facilities.
One of my small secondary schools had second hand music studio equipment, a second hand mini bus etc all passed on by the much bigger secondary down the road when they upgraded.
Another small school I attended for 18 months was shockingly impersonal. The large school my daughter is due to start has already shown more concern and interest in her individual needs and has much better support structures in place.