I think though (particularly with those sizes-I wouldn't regard 900 as majorly small) it will depend on the school and the child, and the year/form they're in.
900 to 1500+ isn't that major a difference to me.
You may find though that with the really big school it's effectively split up into smaller sections, so has the result of for a shy child making it more managable. Dd1's school (10 form entry) they most of the time are in year groups (they have a playground per year group, for example) -even their year group is split in 2 for many things like trips etc. But it has the advantages of lots of opportunities as in a big school.
For dd1, in a lot of ways she would love a small school. But she can very quickly become a big fish in a little pond. She's very shy, painfully shy over some things. But she is very bright and very keen to volunteer and always works hard. So teachers remember her, and she gets a lot of opportunities. The big school has suited her in a way I don't think it suits dd2, or will suit ds, who are nothing like as shy.
I was at a very small school (75 entry) at secondary, and I loved it. However for my dsis, she found that there wasn't anyone in her year she really got on well with. In fact I think she got on better with my friends than her year for the most part. She, although also is shy, I think would have benefitted from a larger school. Because she would have taken the opportunities available and would have been more likely to find people she got on with.