Going from 210 to 300 in one year is a lot. Have they got some new buildings etc to accommodate that?
In my county (quite rural), most schools are based in a larger town, then serve the town and the outlying villages/farms etc. Some are quite small, but most intakes are 240-280. Many of the children come from tiny primaries with 1 form entry or composite classes, but they usually thrive at secondary school.
Personally, I think anything under 100-120 is too small and claustrophobic. As a teenager, you need to be able to find your tribe.
Larger schools can also usually support more subjects/options at GCSE and A-level, provide more clubs and extra-curricular (because there is an interested teacher to run them) and can have better facilities.
In most secondary schools, teachers will teach the same number of pupils/classes, so still have the same number of students to get to know. Forms will be the same size. Larger schools often have assistant heads of year/head of house to help with pastoral care. Pupils may not know teachers who never teach them, or all the SLT, but do they need to?