I've taught two children with SM, and my son was, for a while, a school-indiced selective mute.
I don't think that the setup of classes, or the number of children, or how they group the children academically, is important UNLESS you think your DD's SM is triggered by her feeling bored or alternatively overchallenged. (My DS became selectively mute essentially through terror at the chaos in his classroom - he's on the autistic spectrum, though mild - and so when he moved school it was critical to find a school that placed an emphasis on behaviour and was extremely calm and orderly)
What will matter is the number of staff, particularly support staff, the amount of training each member of the support staff receives each year, the expertise of the SENCo and how pro-active the school is wrt SEN, partnership woth parents, and whether the school is focused on 'the whole child' or narrowly on academics.
The school I was teaching in when I taught one of the selective mutes was exceptionally nurturing. The TAs were experienced, and quite a lot of money was available for their training so one of them immediately went on training related to interventions and support for selective mutism. 'Intervention' time for TAs was timetabled (ie they were not in the class all the time, quite a lot of their time was timetabled for small roups or 1 to 1 interventions) and so specific time was allocated for the TA to work with the child with selective mutism. Parents were encouraged to bring in recordings of what the child had said at home e.g. in response to homework topics, and playing these was a normal part of classroom discussion etc etc.
What you need to know from these schools you are considering is what their approach will be to your DD if she remains selectively mute. You will need to speak to the SENCo in detail. It may be that one or the other has experienced a child with SM. If not, ask them to outline how they think that they would approach it, and also ask them how they typically approach a child with SEN - how long does it take them to bring in specialist help, for example, how many times has a member of staff been on specific training related to a particular condition, what is the movement on and off the SEN register, how are interventions for particular children delivered and are they timetabled for all support staff? You should gradually build up a picture of the school's whole approach to a child who presents on arrival as 'outside the norm' and it is that, not the class setup or the size of the school, that should guide your choice.