Entirely depends on the school and the child.
Downside with a large school can be sensory issues. Even if they are the best behaved children going, a large cohort is noisy and intimidating. I would be looking for sound and space quality of the dining room, main hall, classrooms and hallways. Also whether there are any smaller breakaway play areas in the playground.
On the downside of a small school. I have known a child who was dropped in the the most gorgeous middle class village school and was treated abominably. The school hadn't had anyone with autism before and didn't have a clue. The children were fairly hemogonised and really had very little clue about difference
There is something about large schools being able to hang on to some of there more skilled TA'S even without direct funding whereas a small school may not be able to bridge the gap
I agree with looking for inclusivity in school. Not all of the below will be relevant to your child but they do indicate a school that is thinking about inclusivity and have a degree of knowledge. I would be asking for things like.
Is there a space for children to go if they find the playground is too much? Do they have to go there? is it supervised?
Do any of the children ever wear ear defenders? How do other children react to that.
Do any of the children in the school use writing slopes? (look for them in classes if you can see them the school is already fairly proactive) do you stock pencil grips.
Do you have any connections with the local therapy service. Can it be arranged in necessary
Does your school run any active peer mentoring either academic or playground based.(enthusiasm for these kind of schemes I find often traces back an enthusiastically inclusive SLT and the children are learning about helping the less able members of their cohort)
Do you have community members who come in and help out or read with students (as mentioned above personally I would want an open environment confident that there is nothing to hide)
Do you run any additional handwriting or literacy programs (either on a one to one group or whole school bases
FWIW I really wouldn't be totally reliant on whether they were fazed or not. The special school that were entirely unfazed by DS were unfazed because they did bugger all for the children and failed him miserably. DS' mainstream school were convinced that they couldn't do as well for him but have in fact done fantastically better. What you want is a school that says to you we've looked at your child's needs we are confident we can be of benefit here here and here but are realistic about where there will difficulties too
The thing is you have got to be realistic. If your child's needs can't be met without an ehc then the school should be talking about an ehcp. They won't realistically be able to provide for complex needs in a mainstream environment without it.
Truthfully you are unlikely to know if it is the right place until the end of your first year of school/beginning of the next.
Sometimes it doesn't work out, headteachers change, needs change sometimes it's just not really apparent what the right needs are. I have known a lot of children who swap midway. Even if you "get this decision wrong" (I did), it's not permanent you can redo it it, but you will have a far clearer idea of your child's needs and will be better placed to get it right.