Reception is just like a slightly more structured nursery.
I have a summer-born DD, she started in a primary school nursery at 3.4 with 25 kids all doing a full 9am - 3.30pm day (and lots including her went on to after-school clubs till 6pm) and they all thrived.
She then went to a primary with a 60 intake and moved 3 weeks later to another primary with a 60 intake. No problems at all.
I would choose a bigger school every time over a small school. DD's year have 40 boys and 20 girls. If she had been in a very small village school and the numbers were skewed like that, she'd probably only have a very few other girls to play with which is not ideal.
Starting later in the year means missing out on a lot of the gentle introduction to phonics with the other children and having to play catch-up which is more pressured. Also missing out on making friends, the xmas plays and all the fun things around that.
Having seen the way Y1 works, I would NEVER want to put a child in the position of starting that without a Reception year first. It would be very, very tough for them unless they have been home-schooled to a standard that they could match their classmates.
You will also be amazed at how much she will grow up this year.